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Coupling plasmonic and functional materials provides a promising way to generate multifunctional structures. However, finding plasmonic nanomaterials and elucidating the roles of various geometric and dielectric configurations are tedious. This work describes a combinatorial approach to rapidly exploring and identifying plasmonic heteronanomaterials. Symmetry-broken noble/non-noble metal particle heterojunctions (~100 nanometers) were synthesized on multiwindow silicon chips with silicon nitride membranes. The metal types and the interface locations were controlled to establish a nanoparticle library, where the particle morphology and scattering color can be rapidly screened. By correlating structural data with near- and far-field single-particle spectroscopy data, we found that certain low-energy plasmonic modes could be supported across the heterointerface, while others are localized. Furthermore, we found a series of triangular heteronanoplates stabilized by epitaxial Moiré superlattices, which show strong plasmonic responses despite largely comprising a lossy metal (~70 atomic %). These architectures can become the basis for multifunctional and cost-effective plasmonic devices.

Raman spectroscopic-based biosensing strategies are often complicated by low signal and the presence of multiple chemical species. While surface-enhanced Raman spectroscopy (SERS) nanostructured platforms are able to deliver high quality signals by focusing the electromagnetic field into a tight plasmonic hot-spot, it is not a generally applicable strategy as it often depends on the specific adsorption of the analyte of interest onto the SERS platform. This paper describes a strategy to address this challenge by using surface potential as a physical binding agent in the context of microneedle sensors. We show that the potential-dependent adsorption of different chemical species allows scrutinization of the contributions of different chemical species to the final spectrum, and that the ability to cyclically adsorb and desorb molecules from the surface enables efficient application of multivariate analysis methods. We demonstrate how the strategy can be used to mitigate potentially confounding phenomena, such as surface reactions, competitive adsorption and the presence of molecules with similar structures. In addition, this decomposition helps evaluate criteria to maximize the signal of one molecule with respect to others, offering new opportunities to enhance the measurement of analytes in the presence of interferants., (© 2022. The Author(s).)

Minimally invasive methods for temperature sensing and thermal modulation in living tissues have extensive applications in biological research and clinical care. As alternatives to bioelectronic devices for this purpose, functional nanomaterials that self-assemble into optically active microstructures offer important features in remote sensing, injectability, and compact size. This paper introduces a transient, or bioresorbable, system based on injectable slurries of well-defined microparticles that serve as photopumped lasers with temperature-sensitive emission wavelengths (>4-300 nm °C -1 ). The resulting platforms can act as tissue-embedded thermal sensors and, simultaneously, as distributed vehicles for thermal modulation. Each particle consists of a spherical resonator formed by self-organized cholesteric liquid crystal molecules doped with fluorophores as gain media, encapsulated in thin shells of soft hydrogels that offer adjustable rates of bioresorption through chemical modification. Detailed studies highlight fundamental aspects of these systems including particle sensitivity, lasing threshold, and size. Additional experiments explore functionality as photothermal agents with active temperature feedback (Δ T = 1 °C) and potential routes in remote evaluation of thermal transport properties. Cytotoxicity evaluations support their biocompatibility, and ex vivo demonstrations in Casper fish illustrate their ability to measure temperature within biological tissues with resolution of 0.01 °C. This collective set of results demonstrates a range of multifunctional capabilities in thermal sensing and modulation.

The plasmon-driven chemistry of ferri-/ferrocyanide ions inside surface-enhanced Raman spectroscopy (SERS) active hot spots associated with gold nanoparticle oligomers is studied with continuous wave (CW) pump-probe SERS. By comparing with solution-phase normal Raman spectra, the characteristic spectral variations observed upon 532 nm optical pumping can be attributed to an oxidation process that occurs on the surface species followed by desorption of the oxidized surface species from the gold nanoparticles. Interrogating the plasmon-driven processes over a wide range of temperatures reveals that neither process is purely driven by the thermal effects associated with the optical pumping, and the apparent activation energies of both steps are estimated based on semiquantitative SERS analysis. Our observation identifies a more detailed reaction pathway for this classic model system under considerably simplified reaction conditions, adding to the current mechanistic background for future plasmon-driven chemistry studies and applications.

A novel method for synthesizing and photopatterning colloidal crystals via light-responsive DNA is developed. These crystals are composed of 10-30 nm gold nanoparticles interconnected with azobenzene-modified DNA strands. The photoisomerization of the azobenzene molecules leads to reversible assembly and disassembly of the base-centered cubic (bcc) and face-centered cubic (fcc) crystalline nanoparticle lattices. In addition, UV light is used as a trigger to selectively remove nanoparticles on centimeter-scale thin films of colloidal crystals, allowing them to be photopatterned into preconceived shapes. The design of the azobenzene-modified linking DNA is critical and involves complementary strands, with azobenzene moieties deliberately staggered between the bases that define the complementary code. This results in a tunable wavelength-dependent melting temperature (T m ) window (4.5-15 °C) and one suitable for affecting the desired transformations. In addition to the isomeric state of the azobenzene groups, the size of the particles can be used to modulate the T m window over which these structures are light-responsive., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

The plasmonic properties of tip-substrate composite systems are of vital importance to near-field optical spectroscopy, in particular tip-enhanced Raman spectroscopy (TERS), which enables operando studies of nanoscale chemistry at a single molecule level. The nanocavities formed in the tip-substrate junction also offer a highly tunable platform for studying field-matter interactions at the nanoscale. While the coupled nanoparticle dimer model offers a correct qualitative description of gap-mode plasmon effects, it ignores the full spectrum of multipolar tip plasmon modes and their interaction with surface plasmon polariton (SPP) excitation in the substrate. Herein, we perform the first tip-enhanced Raman excitation spectroscopy (TERES) experiment and use the results, both in ambient and aqueous media, in combination with electrodynamics simulations, to explore the plasmonic response of a Au tip-Au substrate composite system. The gap-mode plasmon features a wide spectral window corresponding to a host of tip plasmon modes interacting with the plasmonic substrate. Simulations of the electric field confinement demonstrate that optimal spatial resolution is achieved when a hybrid plasmon mode that combines a multipolar tip plasmon and a substrate SPP is excited. Nevertheless, a wide spectral window over 1000 nm is available for exciting the tip plasmon with high spatial resolution, which enables the simultaneous resonant detection of different molecular species. This window is robust as a function of tip-substrate distance and tip radius of curvature, indicating that many choices of tips will work, but it is restricted to wavelengths longer than ∼600 nm for the Au tip-Au substrate combination. Other combinations, such as Ag tip-Ag substrate, can access wavelengths as low as 350 nm.

Surface-enhanced Raman spectroscopy (SERS) is a sensitive, chemically specific, and short-time response probing method with significant potential in biomedical sensing. This paper reports the integration of SERS with microneedle arrays as a minimally invasive platform for chemical sensing, with a particular view toward sensing in interstitial fluid (ISF). Microneedle arrays were fabricated from a commercial polymeric adhesive and coated with plasmonically active gold nanorods that were functionalized with the pH-sensitive molecule 4-mercaptobenzoic acid. This sensor can quantitate pH over a range of 5 to 9 and can detect pH levels in an agar gel skin phantom and in human skin in situ. The sensor array is stable and mechanically robust in that it exhibits no loss in SERS activity after multiple punches through an agar gel skin phantom and human skin or after a month-long incubation in phosphate-buffered saline. This work is the first to integrate SERS-active nanoparticles with polymeric microneedle arrays and to demonstrate in situ sensing with this platform.

Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) has been implemented to investigate the structure and activity of iron(II) phthalocyanine (FePc)-a model catalyst for the oxygen reduction reaction (ORR). Using EC-TERS, both reversible change and irreversible degradation to FePc have been observed during ORR. The reversible change in the Raman spectrum of FePc can be related to the FePc molecules that adapt a nonplanar geometry during catalysis. In contrast, the irreversible degradation of FePc is a consequence of FePc demetalation, leading to the subsequent formation of free base phthalocyanine. This observation affirms that FePc demetalation during ORR proceeds via a direct loss of Fe 2+ and that carbon corrosion is not the operative mechanism. Importantly, the FePc demetalation process can be correlated with a loss of ORR activity suggesting that Fe-containing sites are essential for FePc to achieve high catalytic activity. This study establishes EC-TERS as a promising technique for the operando characterization of electrocatalytic reactions at the molecular scale.

Because of the sharp distance dependence of surface-enhanced Raman spectroscopy (SERS), analyte molecules that do not exhibit strong affinity for Au/Ag often elude detection. New methods of integrating such analytes with SERS substrates are required to circumvent this limitation and expand the sensitivity of SERS to new molecules and applications. We communicate here a solution-phase, capture agent-free method of aggregating Au nanospheres in the presence of five neurotransmitters (dopamine, epinephrine, norepinephrine, serotonin, and histamine) and preventing sedimentation by encapsulating the aggregated nanospheres with polyvinylpyrrolidone, thereby trapping the neurotransmitters in close proximity to the Au nanospheres and enabling SER detection. The primary advantages of this physicochemical trapping method, which is generalizable to analytes beyond the scope of this work, are the high signal-to-noise ratio and spectral consistency down to nM levels. Normal Raman spectra and density functional theory calculations corroborate the accuracy of the spectra. Spectra collected over a wide range of concentrations were used to construct adsorption isotherms for all five neurotransmitters, from which adsorption dissociation constants were calculated, spanning from 5.7 × 10 -4 M to 1.7 × 10 -10 M. We expect this method to produce high quality SER spectra of any molecule with an Au affinity known or expected (based on functional groups) to be within that range. Our results have implications for plasmonic detection of these neurotransmitters, particularly for mixtures of those that exhibited disparate Au affinity in our study. We also present evidence that this method produces spectra of sufficient resolution to explore hypotheses related to surface adsorption behavior.

Singlet fission (SF) converts a singlet exciton into two triplet excitons in two or more electronically coupled organic chromophores, which may then be used to increase solar cell efficiency. Many known SF chromophores are unsuitable for device applications due to chemical instability or low triplet state energies. The results described here show that efficient SF occurs in derivatives of 9,10-bis(phenylethynyl)anthracene (BPEA), which is a highly robust and tunable chromophore. Fluoro and methoxy substituents at the 4- and 4'-positions of the BPEA phenyl groups control the intermolecular packing in the crystal structure, which alters the interchromophore electronic coupling, while also changing the SF energetics. The lowest excited singlet state (S 1 ) energy of 4,4'-difluoro-BPEA is higher than that of BPEA so that the increased thermodynamic favorability of SF results in a (16 ± 2 ps) -1 SF rate and a 180% ± 16% triplet yield, which is about an order of magnitude faster than BPEA with a comparable triplet yield. By contrast, 4-fluoro-4'-methoxy-BPEA and 4,4'-dimethoxy-BPEA have slower SF rates, (90 ± 20 ps) -1 and (120 ± 10 ps) -1 , and lower triplet yields, (110 ± 4)% and (168 ± 7)%, respectively, than 4,4'-difluoro-BPEA. These differences are attributed to changes in the crystal structure controlling interchromophore electronic coupling as well as SF energetics in these polycrystalline solids.

Ultrahigh vacuum scanning tunneling microscopy and density functional theory are used to investigate adsorption of oxygen on cobalt phthalocyanine (CoPc), a promising nonprecious metal oxygen reduction catalyst, supported on Ag(111), Cu(111), and Au(111) surfaces at the molecular scale. Four distinct molecular and atomic oxygen adsorption configurations are observed for CoPc supported on Ag(111) surfaces, which are assigned as O 2 /CoPc/Ag(111), O/CoPc/Ag(111), CoPc/(O) 2 /Ag(111), and (O) 2 /CoPc/Ag(111). In contrast, no oxygen adsorption is observed for CoPc supported on Cu(111) and Au(111) surfaces. The results show that for Ag(111), atomic O that is predominantly catalytically produced from the dissociation of molecular O 2 at metal surface step edges is responsible for the observed adsorption configurations. However, Cu(111) binds atomic O too strongly, and Au(111) does not produce atomic O. These results show the active role of the supporting metal surface in facilitating oxygen adsorption on CoPc.

Electrochemical atomic force microscopy tip-enhanced Raman spectroscopy (EC-AFM-TERS) was used for the first time to spatially resolve local heterogeneity in redox behavior on an electrode surface in situ and at the nanoscale. A structurally well-defined Au(111) nanoplate located on a polycrystalline ITO substrate was studied to examine nanoscale redox contrast across the two electrode materials. By monitoring the TERS intensity of adsorbed Nile Blue (NB) molecules on the electrode surface, TERS maps were acquired with different applied potentials. The EC-TERS maps showed a spatial contrast in TERS intensity between Au and ITO. TERS line scans near the edge of a 20 nm-thick Au nanoplate demonstrated a spatial resolution of 81 nm under an applied potential of -0.1 V vs Ag/AgCl. The intensities from the TERS maps at various applied potentials followed Nernstian behavior, and a formal potential ( E 0' ) map was constructed by fitting the TERS intensity at each pixel to the Nernst equation. Clear nanoscale spatial contrast between the Au and ITO regions was observed in the E 0' map. In addition, statistical analysis of the E 0' map identified a statistically significant 4 mV difference in E 0' on Au vs ITO. Electrochemical heterogeneity was also evident in the E 0' distribution, as a bimodal distribution was observed in E 0' on polycrystalline ITO, but not on gold. A direct comparison between an AFM friction image and the E 0' map resolved the electrochemical behavior of individual ITO grains with a spatial resolution of ∼40 nm. The variation in E 0' was attributed to different local surface charges on the ITO grains. Such site-specific electrochemical information with nanoscale spatial and few mV voltage resolutions is not available using ensemble spectroelectrochemical methods. We expect that in situ redox mapping at the nanoscale using EC-AFM-TERS will have a crucial impact on understanding the role of nanoscale surface features in applications such as electrocatalysis.

Nanoparticle encapsulation inside zirconium-based metal-organic frameworks (NP@MOF) is hard to control, and the resulting materials often have nonuniform morphologies with NPs on the external surface of MOFs and NP aggregates inside the MOFs. In this work, we report the controlled encapsulation of gold nanorods (AuNRs) by a scu-topology Zr-MOF, via a room-temperature MOF assembly. This is achieved by functionalizing the AuNRs with poly(ethylene glycol) surface ligands, allowing them to retain colloidal stability in the precursor solution and to seed the MOF growth. Using this approach, we achieve core-shell yields exceeding 99%, tuning the MOF particle size via the solution concentration of AuNRs. The functionality of AuNR@MOFs is demonstrated by using the AuNRs as embedded probes for selective surface-enhanced Raman spectroscopy (SERS). The AuNR@MOFs are able to both take-up or block molecules from the pores, thereby facilitating highly selective sensing at the AuNR ends. This proof-of-principle study serves to present both the outstanding level of control in the synthesis and the high potential for AuNR@Zr-MOF composites for SERS.

Atomic layer deposition (ALD) of TiO 2 was performed in tandem with in situ surface-enhanced Raman spectroscopy (SERS) to monitor changes in the transient surface species across multiple ALD cycles. A self-assembled monolayer of 3-mercaptopropionic acid was used as a capture agent to ensure that nucleation of the titanium precursor (titanium tetraisopropoxide [TTIP]) occurs. Comparisons between the Raman spectra of the neat precursor and the SER spectra of the first ALD cycle of TiO 2 reveal typical ligand exchange chemistry taking place, with self-limiting behavior and intact isopropoxide ligands. However, subsequent cycles show drastically different chemistry, with no isopropoxide ligands remaining at any point during the second and third cycles. Continuous exposure of either TTIP or isopropyl alcohol after the first cycle shows unlimited chemical vapor deposition (CVD)-type growth. Comparisons with alternative precursors (aluminum isopropoxide, titanium tert-butoxide, and titanium propoxide) and DFT calculations reveal that, for the TTIP precursor, isolated TiO 2 sites play a role in the dehydration of off-gassing isopropyl alcohol. The resulting propene then undergoes oligomerization into six-carbon olefins before polymerizing into indistinguishable carbon products that accumulate on the surface. The emergence of the dehydration chemistry is expected to be exclusively the result of these isolated TiO 2 sites and, as such, is expected to occur on other surfaces where TiO 2 ALD is feasible. This work showcases how seemingly innocuous ALD can evolve into a CVD process when the products can participate in various side reactions with newly made surface sites.

Singlet fission (SF) in two or more electronically coupled organic chromophores converts a high-energy singlet exciton into two low-energy triplet excitons, which can be used to increase solar cell efficiency. Many known SF chromophores are unsuitable for device applications due to chemical instability and low triplet state energies. The results described here show that efficient SF occurs in polycrystalline thin films of 9,10-bis(phenylethynyl)anthracene (BPEA), a commercial dye that has singlet and triplet energies of 2.40 and 1.11 eV, respectively, in the solid state. BPEA crystallizes into two polymorphs with space groups C2/ c and Pbcn, which undergo SF with k SFA = (109 ± 4 ps) -1 and k SFB = (490 ± 10 ps) -1 , respectively. The high triplet energy and efficient SF evidenced from the 180 ± 20% triplet yield make BPEA a promising candidate for enhancing solar cell performance.

Jarosite, a mineral with a kagomé lattice, displays magnetic frustration yet orders magnetically below 65 K. As magnetic frustration can engender exotic physical properties, understanding the complex magnetism of jarosite comprises a multidecade interdisciplinary challenge. Unraveling the nature of the disparate magnetic coupling interactions that lead to magnetic order in jarosite remains an open question. Specifically, there is no observed trend in the interlayer spacing with magnetic order. Similarly, the relationship between metal-ligand bond distance and magnetic order remains uninvestigated. Here, we use applied pressure to smoothly vary jarosite's structure without manipulating the chemical composition, enabling a chemically invariant structure-function study. Using single-crystal and powder X-ray diffraction, we show that high applied pressures alter both the interlayer spacing and the metal-ligand bond distances. By harnessing a suite of magnetic techniques under pressure, including SQUID-based magnetometry, time-resolved synchrotron Mössbauer spectroscopy, and X-ray magnetic circular dichroism, we construct the magnetic phase diagram for jarosite up to 40 GPa. Notably, we demonstrate that the magnetic ordering temperature increases dramatically to 240 K at the highest pressures. Additionally, we conduct X-ray emission spectroscopy, Mössbauer spectroscopy, and UV-visible absorption spectroscopy experiments to comprehensively map the magnetic and electronic structures of jarosite at high pressure. We use these maps to construct chemically pure magnetostructural correlations which fully explain the nature and role of the disparate magnetic coupling interactions in jarosite.

Continuous wave (CW) pump-probe surface-enhanced Raman spectroscopy (SERS) is used to examine a range of plasmon-driven chemical behavior in the molecular SERS signal of trans-1,2-bis(4-pyridyl)ethylene (BPE) adsorbed on individual Au nanosphere oligomers (viz., dimers, trimers, tetramers, etc.). Well-defined new transient modes are caused by high fluence CW pumping at 532 nm and are monitored on the seconds time scale using a low intensity CW probe field at 785 nm. Comparison of time-dependent density functional theory (TD-DFT) calculations with the experimental data leads to the conclusion that three independent chemical processes are operative: (1) plasmon-driven electron transfer to form the BPE anion radical; (2) BPE hopping between two adsorption sites; and (3) trans-to- cis-BPE isomerization. Resonance Raman and electron paramagnetic resonance (EPR) spectroscopy measurements provide further substantiation for the observation of an anion radical species formed via a plasmon-driven electron transfer reaction. Applications of these findings will greatly impact the design of novel plasmonic devices with the future ability to harness new and efficient energetic pathways for both chemical transformation and photocatalysis at the nanoscale level.

Chemically modified tips in scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been used to improve the imaging resolution or provide richer chemical information, mostly in ultrahigh vacuum (UHV) environments. Tip-enhanced Raman spectroscopy (TERS) is a nanoscale spectroscopic technique that already provides chemical information and can provide subnanometer spatial resolution. Chemical modification of TERS tips has mainly been focused on increasing their lifetimes for ambient and in situ experiments. Under UHV conditions, chemical functionalization has recently been carried out to increase the amount of chemical information provided by TERS. However, this strategy has not yet been extended to in situ electrochemical (EC)-TERS studies. The independent control of the tip and sample potentials offered by EC-STM allows us to prove the in situ functionalization of a tip in EC-STM-TERS. Additionally, the Raman response of chemically modified TERS tips can be switched on and off at will, which makes EC-STM-TERS an ideal platform for the development of in situ chemical probes on the nanoscale.

Ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) is used to investigate adsorption of molecular oxygen (O 2 ) on cobalt(II) phthalocyanine (CoPc) supported on Ag(111) single crystal surfaces, which is the initial step for the oxygen reduction reaction (ORR) using metal Pc catalysts. Two adsorption configurations are primarily observed, assigned as O 2 /CoPc/Ag(111) and O/CoPc/Ag(111) based on scanning tunneling microscopy (STM) imaging, TERS, isotopologue substitution, and density functional theory (DFT) calculations. Distinct vibrational features are observed for different adsorption configurations such as the 18 O- 18 O stretching frequency at 1151 cm -1 for O 2 /CoPc/Ag(111), and Co- 16 O and Co- 18 O vibrational frequencies at 661 and 623 cm -1 , respectively, for O/CoPc/Ag(111). DFT calculations show vibrational mode coupling of O-O and Co-O vibrations to the Pc ring, resulting in different symmetries of oxygen-related normal modes. This study establishes UHV-TERS as a chemically sensitive tool for probing catalytic systems at the molecular scale.

Ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) combines the atomic-scale imaging capability of scanning probe microscopy with the single-molecule chemical sensitivity and structural specificity of surface-enhanced Raman spectroscopy. Here, we use these techniques in combination with theory to reveal insights into the influence of intermolecular interactions on the vibrational spectra of a N-N'-bis(2,6-diisopropylphenyl)-perylene-3,4:9,10-bis(dicarboximide) (PDI) self-assembled monolayer adsorbed on single-crystal Ag substrates at room temperature. In particular, we have revealed the lifting of a vibrational degeneracy of a mode of PDI on Ag(111) and Ag(100) surfaces, with the most strongly perturbed mode being that associated with the largest vibrational amplitude on the periphery of the molecule. This work demonstrates that UHV-TERS enables direct measurement of molecule-molecule interaction at nanoscale. We anticipate that this information will advance the fundamental understanding of the most important effect of intermolecular interactions on the vibrational modes of surface-bound molecules.

Four decades on, surface-enhanced Raman spectroscopy (SERS) continues to be a vibrant field of research that is growing (approximately) exponentially in scope and applicability while pushing at the ultimate limits of sensitivity, spatial resolution, and time resolution. This introductory paper discusses some aspects related to all four of the themes for this Faraday Discussion. First, the wavelength-scanned SERS excitation spectroscopy (WS-SERES) of single nanosphere oligomers (viz., dimers, trimers, etc.), the distance dependence of SERS, the magnitude of the chemical enhancement mechanism, and the progress toward developing surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS) are discussed. Second, our efforts to develop a continuous, minimally invasive, in vivo glucose sensor based on SERS are highlighted. Third, some aspects of our recent work in single molecule SERS and the translation of that effort to ångstrom-scale spatial resolution in ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) and single molecule electrochemistry using electrochemical (EC)-TERS will be presented. Finally, we provide an overview of analytical SERS with our viewpoints on SERS substrates, approaches to address the analyte generality problem (i.e. target molecules that do not spontaneously adsorb and/or have Raman cross sections <10 -29 cm 2 sr -1 ), SERS for catalysis, and deep UV-SERS.

We clarify mechanistic questions regarding plasmon-driven chemistry and nanoscale photocatalysis within optically confined near-field plasmonic systems. Using surface-enhanced Raman scattering (SERS), we directly monitor the photoinduced reaction dynamics of 4,4'-bipyridine molecules, localized in plasmonic hot spots within individual gold nanosphere oligomers. Our experiment generates surface electrons from the gold nanoparticle using an intense off-molecular resonance continuous wave pump field, and detects radical anion products via SERS. This is done by adopting a dual-wavelength spectroscopic approach. Empirical evidence of plasmon-driven electron transfer is provided for the first time by direct detection of the 4,4'-bipyridine radical anion species localized in the plasmonic hot spots of individual gold nanosphere oligomers, corroborated by open-shell density functional theory calculations. An isotopologue approach using both protonated and deuterated 4,4'-bipyridine molecules demonstrates the single molecule response of plasmon-driven electron transfer occurring in single nanosphere oligomer systems with a 3% yield, a phenomenon unobserved in ensemble measurements under analogous experimental conditions. This mechanism has broad applicability to using nanoscale chemical reactors for surface redox reactions on the subnanometer scale.

Native electron capture dissociation (NECD) is a process during which proteins undergo fragmentation similar to that from radical dissociation methods, but without the addition of exogenous electrons. However, after three initial reports of NECD from the cytochrome c dimer complex, no further evidence of the effect has been published. Here, we report NECD behavior from horse spleen ferritin, a ∼490 kDa protein complex ∼20-fold larger than the previously studied cytochrome c dimer. Application of front-end infrared excitation (FIRE) in conjunction with low- and high-m/z quadrupole isolation and collisionally activated dissociation (CAD) provides new insights into the NECD mechanism. Additionally, activation of the intact complex in either the electrospray droplet or the gas phase produced c-type fragment ions. Similar to the previously reported results on cytochrome c, these fragment ions form near residues known to interact with iron atoms in solution. By mapping the location of backbone cleavages associated with c-type ions onto the crystal structure, we are able to characterize two distinct iron binding channels that facilitate iron ion transport into the core of the complex. The resulting pathways are in good agreement with previously reported results for iron binding sites in mammalian ferritin.

We present the first multiwavelength surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS) study, as well as the first observation of anti-Stokes vibrational features in SE-FSRS spectra. We compare stimulated Raman loss (SRL) and stimulated Raman gain (SRG) signals at three pump wavelengths chosen to sample different portions of nanoparticle aggregate localized surface plasmon resonances. The SE-FSRS signals exhibit similar signal magnitudes in the SRL or SRG regions of the spectra regardless of Raman pump or probe wavelength. The spectral lineshapes, however, differ dramatically with excitation wavelengths. The observed trends in spectral line shape show a strong dependence on the relative position of the excitation fields with respect to the plasmon resonance but do not match predictions from any existing SE-FSRS theory. These results suggest the need for further theoretical efforts with complementary experimental studies of individual aggregates to remove the effects of inherent ensemble averaging.

Femtosecond stimulated Raman spectroscopy (FSRS) is a vibrational spectroscopy technique that has been used in a wide variety of applications: from transient vibrational signature tracking to amplifying weak normal Raman scattering signals. Presented here is an application of FSRS to quantify the differential Raman scattering cross sections (DRSCs) of glucose. In using FSRS to determine the DRSCs of multiple glucose vibrational modes, we demonstrate the applicability of both stimulated Raman loss (SRL) spectroscopy and stimulated Raman gain (SRG) FSRS. Using the two analogous FSRS techniques, SRG and SRL, we determine that the DRSCs of glucose excited at 514.5 nm range from a low of 5.0 ± 1.1 × 10 -30 to a high of 8.9 ± 0.9 × 10 -30 cm 2 molecule -1 sr -1 . This work establishes both the compatibility of SRL for measuring DRSCs and values for the DRSC of multiple vibrational modes of glucose.

Surface-enhanced Raman scattering (SERS) spectroscopy has evolved into a cross-disciplinary analytical technique by unveiling relevant chemical, biological, material, and structural information. The focus of this review is on two critical properties for successfully expanding applications of SERS spectroscopy: quality of the plasmonic substrate and molecule localization to the substrate. In this review, we discuss recent work on quantifying SERS distance dependence, key factors for substrate characterization and performance evaluation, expansion of SERS applications through substrate development for UV plasmonics and short-distance capture strategies for optimizing analyte-surface structures. After surveying the recent developments of these seemingly disparate fields, we suggest new research directions that may originate from a synergistic blend of all the herein discussed topics. Finally, we discuss major challenges and open questions related to the application of SERS for understanding of chemical processes at the nanoscale, with special interest on in situ catalysts and biosensing.

Single-molecule (SM) surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) have emerged as analytical techniques for characterizing molecular systems in nanoscale environments. SERS and TERS use plasmonically enhanced Raman scattering to characterize the chemical information on single molecules. Additionally, TERS can image single molecules with subnanometer spatial resolution. In this review, we cover the development and history of SERS and TERS, including the concept of SERS hot spots and the plasmonic nanostructures necessary for SM detection, the past and current methodologies for verifying SMSERS, and investigations into understanding the signal heterogeneities observed with SMSERS. Moving on to TERS, we cover tip fabrication and the physical origins of the subnanometer spatial resolution. Then, we highlight recent advances of SMSERS and TERS in fields such as electrochemistry, catalysis, and SM electronics, which all benefit from the vibrational characterization of single molecules. SMSERS and TERS provide new insights on molecular behavior that would otherwise be obscured in an ensemble-averaged measurement.

Errors in intravenous (IV) drug therapies can cause human harm and even death. There are limited label-free methods that can sensitively monitor the identity and quantity of the drug being administered. Normal Raman spectroscopy (NRS) provides a modestly sensitive, label-free, and completely noninvasive means of IV drug sensing. In the case that the analyte cannot be detected within its clinical range with Raman, a label-free surface-enhanced Raman spectroscopy (SERS) approach can be implemented to detect the analyte of interest. In this work, we demonstrate two individual cases where we use NRS and electrochemical SERS (EC-SERS) to detect IV therapy analytes within their clinically relevant ranges. We implement NRS to detect gentamicin, a commonly IV-administered antibiotic and EC-SERS to detect dobutamine, a drug commonly administered after heart surgery. In particular, dobutamine detection with EC-SERS was found to have a limit of detection 4 orders of magnitude below its clinical range, highlighting the excellent sensitivity of SERS. We also demonstrate the use of hand-held Raman instrumentation for NRS and EC-SERS, showing that Raman is a highly sensitive technique that is readily applicable in a clinical setting.

Operando surface-enhanced Raman spectroscopy (SERS) was used to successfully identify hitherto unknown dimeric methylalumina surface species during atomic layer deposition (ALD) on a silver surface. Vibrational modes associated with the bridging moieties of both trimethylaluminum (TMA) and dimethylaluminum chloride (DMACl) surface species were found during ALD. The appropriate monomer vibrational modes were found to be absent as a result of the selective nature of SERS. Density functional theory (DFT) calculations were also performed to locate and identify the expected vibrational modes. An operando localized surface plasmon resonance (LSPR) spectrometer was utilized to account for changes in SER signal as a function of the number of ALD cycles. DMACl surface species were unable to be measured after multiple ALD cycles as a result of a loss in SERS enhancement and shift in LSPR. This work highlights how operando optical spectroscopy by SERS and LSPR scattering are useful for probing the identity and structure of the surface species involved in ALD and, ultimately, catalytic reactions on these support materials.

Electrochemical atomic force microscopy tip-enhanced Raman spectroscopy (EC-AFM-TERS) was employed for the first time to observe nanoscale spatial variations in the formal potential, E 0' , of a surface-bound redox couple. TERS cyclic voltammograms (TERS CVs) of single Nile Blue (NB) molecules were acquired at different locations spaced 5-10 nm apart on an indium tin oxide (ITO) electrode. Analysis of TERS CVs at different coverages was used to verify the observation of single-molecule electrochemistry. The resulting TERS CVs were fit to the Laviron model for surface-bound electroactive species to quantitatively extract the formal potential E 0' at each spatial location. Histograms of single-molecule E 0' at each coverage indicate that the electrochemical behavior of the cationic oxidized species is less sensitive to local environment than the neutral reduced species. This information is not accessible using purely electrochemical methods or ensemble spectroelectrochemical measurements. We anticipate that quantitative modeling and measurement of site-specific electrochemistry with EC-AFM-TERS will have a profound impact on our understanding of the role of nanoscale electrode heterogeneity in applications such as electrocatalysis, biological electron transfer, and energy production and storage.

We report here the first fabrication of aluminum film-over nanosphere (AlFON) substrates for UV surface-enhanced resonance Raman scattering (UVSERRS) at the deepest UV wavelength used to date (λ ex = 229 nm). We characterize the AlFONs fabricated with two different support microsphere sizes using localized surface plasmon resonance spectroscopy, electron microscopy, SERRS of adenine, tris(bipyridine)ruthenium(II), and trans-1,2-bis(4-pyridyl)-ethylene, SERS of 6-mercapto-1-hexanol (as a nonresonant molecule), and dielectric function analysis. We find that AlFONs fabricated with the 210 nm microspheres generate an enhancement factor of approximately 10 4-5 , which combined with resonance enhancement of the adsorbates provides enhancement factors greater than 10 6 . These experimental results are supported by theoretical analysis of the dielectric function. Hence our results demonstrate the advantages of using AlFON substrates for deep UVSERRS enhancement and contribute to broadening the SERS application range with tunable and affordable substrates.

Tip-enhanced Raman spectroscopy (TERS) combines the ability of scanning probe microscopy (SPM) to resolve atomic-scale surface features with the single-molecule chemical sensitivity of surface-enhanced Raman spectroscopy (SERS). Here, we report additional insights into the nature of the conformational dynamics of a free-base porphyrin at room temperature adsorbed on a metal surface. We have interrogated the conformational switch between two metastable surface-mediated isomers of meso-tetrakis(3,5-ditertiarybutylphenyl)-porphyrin (H 2 TBPP) on a Cu(111) surface. At room temperature, the barrier between the porphyrin ring buckled up/down conformations of the H 2 TBPP-Cu(111) system is easily overcome, and a 2.6 Å lateral resolution by simultaneous TERS and STM analysis is achieved under ultrahigh vacuum (UHV) conditions. This work demonstrates the first UHV-TERS on Cu(111) and shows TERS can unambiguously distinguish the conformational differences between neighboring molecules with Ångstrom-scale spatial resolution, thereby establishing it as a leading method for the study of metal-adsorbate interactions.

Surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS) is an ultrafast Raman technique that combines the sensitivity of surface-enhanced Raman scattering with the temporal resolution of femtosecond stimulated Raman spectroscopy (FSRS). Here, we present the first successful implementation of SE-FSRS using a 1 MHz amplified femtosecond laser system. We compare SE-FSRS and FSRS spectra measured at 1 MHz and 100 kHz using both equal pump average powers and equal pump energies to demonstrate that higher repetition rates allow spectra with higher signal-to-noise ratios to be obtained at lower pulse energies, a significant advance in the implementation of SE-FSRS. The ability to use lower pulse energies significantly mitigates sample damage that results from plasmonic enhancement of high-energy ultrafast pulses. As a result of the improvements to SE-FSRS developed in this Letter, we believe that SE-FSRS is now poised to become a powerful tool for studying the dynamics of plasmonic materials and adsorbates thereon.

Optically heterodyne-detected femtosecond Raman-induced Kerr-effect spectroscopy (OHD-FRIKES) was observed in neat cyclohexane. In this Letter, an examination of the effect of the Raman pump ellipticity on the multiplex OHD-FRIKES spectra is discussed. The Raman pump ellipticity scanned OHD-FRIKES results reproduce anomalous observables from previous OHD-FRIKES experiments and suggest new methods of tracking transient vibrational mode polarization in complex systems.

This paper demonstrates the direct sensing of glucose at physiologically relevant concentrations with surface-enhanced Raman spectroscopy (SERS) on gold film-over-nanosphere (AuFON) substrates functionalized with bisboronic acid receptors. The combination of selectivity in the bisboronic acid receptor and spectral resolution in the SERS data allow the sensors to resolve glucose in high backgrounds of fructose and, in combination with multivariate statistical analysis, detect glucose accurately in the 1-10 mM range. Computational modeling supports assignments of the normal modes and vibrational frequencies for the monoboronic acid base of our bisboronic acids, glucose and fructose. These results are promising for the use of bisboronic acids as receptors in SERS-based in vivo glucose monitoring sensors.

The chemical sensitivity of surface-enhanced Raman spectroscopy (SERS) methodologies allows for the investigation of heterogeneous chemical reactions with high sensitivity. Specifically, SERS methodologies are well-suited to study electron transfer (ET) reactions, which lie at the heart of numerous fundamental processes: electrocatalysis, solar energy conversion, energy storage in batteries, and biological events such as photosynthesis. Heterogeneous ET reactions are commonly monitored by electrochemical methods such as cyclic voltammetry, observing billions of electrochemical events per second. Since the first proof of detecting single molecules by redox cycling, there has been growing interest in examining electrochemistry at the nanoscale and single-molecule levels. Doing so unravels details that would otherwise be obscured by an ensemble experiment. The use of optical spectroscopies, such as SERS, to elucidate nanoscale electrochemical behavior is an attractive alternative to traditional approaches such as scanning electrochemical microscopy (SECM). While techniques such as single-molecule fluorescence or electrogenerated chemiluminescence have been used to optically monitor electrochemical events, SERS methodologies, in particular, have shown great promise for exploring electrochemistry at the nanoscale. SERS is ideally suited to study nanoscale electrochemistry because the Raman-enhancing metallic, nanoscale substrate duly serves as the working electrode material. Moreover, SERS has the ability to directly probe single molecules without redox cycling and can achieve nanoscale spatial resolution in combination with super-resolution or scanning probe microscopies. This Account summarizes the latest progress from the Van Duyne and Willets groups toward understanding nanoelectrochemistry using Raman spectroscopic methodologies. The first half of this Account highlights three techniques that have been recently used to probe few- or single-molecule electrochemical events: single-molecule SERS (SMSERS), superlocalization SERS imaging, and tip-enhanced Raman spectroscopy (TERS). While all of the studies we discuss probe model redox dye systems, the experiments described herein push the study of nanoscale electrochemistry toward the fundamental limit, in terms of both chemical sensitivity and spatial resolution. The second half of this Account discusses current experimental strategies for studying nanoelectrochemistry with SERS techniques, which includes relevant electrochemically and optically active molecules, substrates, and substrate functionalization methods. In particular, we highlight the wide variety of SERS-active substrates and optically active molecules that can be implemented for EC-SERS, as well as the need to carefully characterize both the electrochemistry and resultant EC-SERS response of each new redox-active molecule studied. Finally, we conclude this Account with our perspective on the future directions of studying nanoscale electrochemistry with SERS/TERS, which includes the integration of SECM with TERS and the use of theoretical methods to further describe the fundamental intricacies of single-molecule, single-site electrochemistry at the nanoscale.

We present a coupled wave semiclassical theory to describe plasmonic enhancement effects in surface-enhanced femtosecond stimulated Raman scattering (SE-FSRS). A key result is that the plasmon enhanced fields which drive the vibrational equation of motion for each normal mode results in dispersive lineshapes in the SE-FSRS spectrum. This result, which reproduces experimental lineshapes, demonstrates that plasmon-enhanced stimulated Raman methods provide unique sensitivity to a plasmonic response. Our derived SE-FSRS theory shows a plasmonic enhancement of |gpu|(2)ImχR(ω)gst (2)/ImχR(ω), where |gpu|(2) is the absolute square of the plasmonic enhancement from the Raman pump, χR(ω) is the Raman susceptibility, and gst is the plasmonic enhancement of the Stokes field in SE-FSRS. We conclude with a discussion on potential future experimental and theoretical directions for the field of plasmonically enhanced coherent Raman scattering.

We report a systematic study performed in ultrahigh vacuum designed to identify the laser excitation regimes in which plasmonically enhanced ultrashort pulses may be used to nondestructively probe surface-bound molecules. A nondestructive, continuous-wave spectroscopic probe is used to monitor the effects of four different femtosecond- and picosecond-pulsed beams on the SER signals emanating from molecular analytes residing within plasmonically enhanced fields. We identify the roles of plasmonic amplification and alignment with a molecular electronic transition on the observed changes in the SER signals. Our results indicate that overlap of the laser wavelength with the plasmon resonance is the dominant contributor to signal degradation. In addition, signal loss for a given irradiation condition is observed only for molecules residing in hot spots above a threshold enhancement. Identification of suitable laser energy density ranges demonstrates the importance of considering these parameters when implementing SERS in the presence of pulsed irradiation.

We present a high-resolution distance dependence study of surface-enhanced Raman scattering (SERS) enabled by atomic layer deposition (ALD) at 55 and 100 °C. ALD is used to deposit monolayers of Al2O3 on bare silver film over nanospheres (AgFONs) and AgFONs functionalized with self-assembled monolayers. Operando SERS is used to measure the intensities of the Al-CH3 and C-H stretches from trimethylaluminum (TMA) as a function of distance from the AgFON surface. This study clearly demonstrates that SERS on AgFON substrates displays both a short- and long-range nanometer scale distance dependence. Excellent agreement is obtained between these experiments and theory that incorporates both short-range and long-range terms. This is a high-resolution operando SERS distance dependence study performed in one integrated experiment using ALD Al2O3 as the spacer layer and Raman label simultaneously. The long-range SERS distance dependence should make it possible to detect chemisorbed surface species located as far as ∼3 nm from the AgFON substrate and will provide new insight into the surface chemistry of ALD and catalytic reactions.

This perspective presents recent developments in the application of surface-enhanced Raman spectroscopy (SERS) to biosensing, with a focus on in vivo diagnostics. We describe the concepts and methodologies developed to date and the target analytes that can be detected. We also discuss how SERS has evolved from a "point-and-shoot" stand-alone technique in an analytical chemistry laboratory to an integrated quantitative analytical tool for multimodal imaging diagnostics. Finally, we offer a guide to the future of SERS in the context of clinical diagnostics.

Nanoscale chemical imaging of a dynamic molecular phase boundary has broad implications for a range of problems in catalysis, surface science, and molecular electronics. While scanning probe microscopy (SPM) is commonly used to study molecular phase boundaries, its information content can be severely compromised by surface diffusion, irregular packing, or three-dimensional adsorbate geometry. Here, we demonstrate the simultaneous chemical and structural analysis of N-N'-bis(2,6-diisopropylphenyl)-1,7-(4'-t-butylphenoxy)perylene-3,4:9,10-bis(dicarboximide) (PPDI) molecules by UHV tip-enhanced Raman spectroscopy. Both condensed and diffusing domains of PPDI coexist on Ag(100) at room temperature. Through comparison with time-dependent density functional theory simulations, we unravel the orientation of PPDI molecules at the dynamic molecular domain boundary with unprecedented ∼4 nm spatial resolution.

We demonstrate that incorporation of a redox-active benzoquinoid ligand into a one-dimensional chain compound can give rise to a material that exhibits simultaneous solid-state redox switching of optical, magnetic, and electronic properties. Metalation of the ligand 4,5-bis(pyridine-2-carboxamido)-1,2-catechol ((N,O)LH4) with Mn(III) affords the chain compound Mn((N,O)L)(DMSO). Structural and spectroscopic analysis of this compound show the presence of Mn(II) centers bridged by (N,O)L(2-) ligands, resulting partially from a spontaneous ligand-to-metal electron transfer. Upon soaking in a solution of the reductant Cp2Co, Mn((N,O)L)(DMSO) undergoes a ligand-centered solid-state reduction to [Mn((N,O)L)](-), as revealed by a suite of techniques, including Raman and X-ray absorption spectroscopy. The ligand-based reduction engenders a dramatic modulation of the physical properties of the chain compound. An electrochromic response, evidenced by a color change from dark green to dark purple is accompanied by a nearly 40-fold increase in magnetic coupling strength, from J = -0.38(1) to -15.6(2) cm(-1), and a 10,000-fold increase in electronic conductivity, from σ = 2.33(1) × 10(-12) S/cm (Ea = 0.64(1) eV) to 8.61(1) × 10(-8) S/cm (Ea = 0.39(1) eV). Importantly, the chemical reduction is reversible: treatment of the reduced compound with [Cp2Fe](+) regenerates the oxidized chain. Taken together, these results highlight the ability of benzoquinoid ligands to facilitate solid-state ligand-based redox reactions in nonporous coordination solids, giving rise to reversible switching of optical properties, magnetic exchange interactions, and electronic conductivity.

Ultrafast surface-enhanced Raman spectroscopy (SERS) has the potential to study molecular dynamics near plasmonic surfaces to better understand plasmon-mediated chemical reactions such as plasmonically-enhanced photocatalytic or photovoltaic processes. This review discusses the combination of ultrafast Raman spectroscopic techniques with plasmonic substrates for high temporal resolution, high sensitivity, and high spatial resolution vibrational spectroscopy. First, we introduce background information relevant to ultrafast SERS: the mechanisms of surface enhancement in Raman scattering, the characterization of plasmonic materials with ultrafast techniques, and early complementary techniques to study molecule-plasmon interactions. We then discuss recent advances in surface-enhanced Raman spectroscopies with ultrafast pulses with a focus on the study of molecule-plasmon coupling and molecular dynamics with high sensitivity. We also highlight the challenges faced by this field by the potential damage caused by concentrated, highly energetic pulsed fields in plasmonic hotspots, and finally the potential for future ultrafast SERS studies.

Simplicity and low cost has positioned inkjet paper- and fabric-based 3D substrates as two of the most commonly used surface-enhanced Raman spectroscopy (SERS) platforms for the detection and the identification of chemical and biological analytes down to the nanogram and femtogram levels. The relationship between far-field and near-field properties of these 3D SERS platforms remains poorly understood and warrants more detailed characterization. Here, we investigate the extremely weak optical scattering observed from commercial and home-fabricated paper-, as well as fabric-based 3D SERS substrates. Using wavelength scanned surface-enhanced Raman excitation spectroscopy (WS-SERES) and finite-difference time-domain (FDTD) calculations we were able to determine their near-field SERS properties and correlate them with morphological and far-field properties. It was found that nanoparticle dimers, trimers, and higher order nanoparticle clusters primarily determine the near-field properties of these substrates. At the same time, the far-field response of 3D SERS substrates either originates primarily from the monomers or cannot be clearly defined. Using FDTD we demonstrate that LSPR bands of nanoparticle aggregates near perfectly overlap with the maxima of the near-field surface-enhanced Raman scattering responses of the 3D SERS substrates. This behaviour of far-field spectroscopic properties and near-field surface-enhanced Raman scattering has not been previously observed for 2D SERS substrates, known as nanorod arrays. The combination of these analytical approaches provides a full spectroscopic characterization of 3D SERS substrates, while FDTD simulation can be used to design new 3D SERS substrates with tailored spectral characteristics.

Protonated heterocyclic amines, such as pyridinium, have been utilized as catalysts in the electrocatalytic reduction of carbon dioxide. While these represent a new and exciting class of electrocatalysts, the details of the mechanism and faradaic processes occurring in solution are unclear. We report a series of cyclic voltammetry experiments involving Pt, Ag, Au, and Cu electrodes, under both aqueous and nonaqueous conditions, directed towards gaining an improved mechanistic understanding of pyridinium electrochemistry. Surface-enhanced Raman (SER) spectroelectrochemistry was also performed on Cu film-over-nanosphere electrodes in order to identify adsorbed species. It was found that the reduction potential of pyridinium (-0.58 V vs. SCE) and its electrochemical reversibility are unique features of platinum electrodes. In contrast, the reduction potentials on Ag, Au, and Cu electrodes are ∼400 mV more negative than Pt in both the presence and the absence of CO2. SER spectroelectrochemistry of pyridinium solutions shows no evidence for a pyridinium radical or a pyridinium ion. Increased cathodic current in the presence of CO2 is only detected at scan rates less than 10 mV s(-1) in aqueous solutions. The addition of CO2 resulted in a shift in the potential for the hydrogen evolution reaction. Pyridinium electrochemistry was observed under nonaqueous conditions; however no increase in cathodic current was observed when CO2 was added to the solution. Based on this set of results it is concluded that the reduction potential of pyridinium is surface dependent, CO2 acts as a pseudo-reserve of H(+), and pyridinium and CO2 create an alternative mechanism for hydrogen evolution.

The incorporation of tetraoxolene radical bridging ligands into a microporous magnetic solid is demonstrated. Metalation of the redox-active bridging ligand 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH2) with Fe(II) affords the solid (Me2NH2)2[Fe2L3]·2H2O·6DMF. Analysis of X-ray diffraction, Raman spectra, and Mössbauer spectra confirm the presence of Fe(III) centers with mixed-valence ligands of the form (L3)(8-) that result from a spontaneous electron transfer from Fe(II) to L(2-). Upon removal of DMF and H2O solvent molecules, the compound undergoes a slight structural distortion to give the desolvated phase (Me2NH2)2[Fe2L3], and a fit to N2 adsorption data of this activated compound gives a BET surface area of 885(105) m(2)/g. Dc magnetic susceptibility measurements reveal a spontaneous magnetization below 80 and 26 K for the solvated and the activated solids, respectively, with magnetic hysteresis up to 60 and 20 K. These results highlight the ability of redox-active tetraoxolene ligands to support the formation of a microporous magnet and provide the first example of a structurally characterized extended solid that contains tetraoxolene radical ligands.