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A liquid-based surface-enhanced Raman spectroscopy assay termed PSALM is developed for the selective sensing of neurotransmitters (NTs) with a limit of detection below the physiological range of NT concentrations in urine. This assay is formed by quick and simple nanoparticle (NP) "mix-and-measure" protocols, in which FeIII bridges NTs and gold NPs inside the sensing hotspots. Detection limits of NTs from PreNP PSALM are significantly lower than those of PostNP PSALM, when urine is pretreated by affinity separation. Optimized PSALM enables the long-term monitoring of NT variation in urine in conventional settings for the first time, allowing the development of NTs as predictive or correlative biomarkers for clinical diagnosis.

Gold nanoparticles used in many types of nanostructure are mostly stabilized by citrate ligands. Fully understanding their dynamic surface chemistry is thus essential for applications, particularly since aging is frequently a problem. Using surface-enhanced Raman spectroscopy in conjunction with density functional theory calculations, we are able to determine Au-citrate coordination in liquid with minimal invasiveness. We show that citrate coordination is mostly bidentate and simply controlled by its protonation state. More complex binding motifs are caused by interfering chloride ions and gold adatoms. With increasing age of stored gold nanoparticle suspensions, gold adatoms are found to move atop the Au facets and bind to an additional terminal carboxylate of the citrate. Aged nanoparticles are fully refreshed by removing these adatoms, using etching and subsequent boiling of the gold nanoparticles.

Metal/organic-molecule interactions underpin many key chemistries but occur on sub-nm scales where nanoscale visualisation techniques tend to average over heterogeneous distributions. Single molecule imaging techniques at the atomic scale have found it challenging to track chemical behaviour under ambient conditions. Surface-enhanced Raman spectroscopy can optically monitor the vibrations of single molecules but understanding is limited by the complexity of spectra and mismatch between theory and experiment. We demonstrate that spectra from an optically generated metallic adatom near a molecule of interest can be inverted into dynamic sub-Å metal-molecule interactions using a comprehensive model, revealing anomalous diffusion of a single atom. Transient metal-organic coordination bonds chemically perturb molecular functional groups > 10 bonds away. With continuous improvements in computational methods for modelling large and complex molecular systems, this technique will become increasingly applicable to accurately tracking more complex chemistries., We acknowledge financial support from EPSRC grant EP/G060649/1, EP/L027151/1, EP/G037221/1, EP/R013012/1, EPSRC NanoDTC, and EU grant THOR 829067 and ERC starting grant BioNet 757850. B.d.N. acknowledges support from the Leverhulme Trust and Isaac Newton Trust. We acknowledge use of the Rosalind computing facility at King’s College London. We are grateful to the UK Materials and Molecular Modelling Hub for computational resources, which is partially funded by EPSRC 397 (EP/P020194/1).

Irreproducibility in surface-enhanced Raman spectroscopy (SERS) due to variability among substrates is a source of recurrent debate within the field. It is regarded as a major hurdle towards the widespread adoption of SERS as a sensing platform. Most of the literature focused on developing substrates for various applications considers reproducibility of lower importance. Here, we address and analyse the sources of this irreproducibility in order to show how these can be minimised. We apply our findings to a simple substrate demonstrating reproducible SERS measurements with relative standard deviations well below 1% between different batches and days. Identifying the sources of irreproducibility and understanding how to reduce these can aid in the transition of SERS from the lab to real world applications.

External-stimuli controlled reversible formation of radical species is of great interest for synthetic and supramolecular chemistry, molecular machinery, as well as emerging technologies ranging from (photo)catalysis and photovoltaics to nanomedicine. Here we show a novel hybrid colloidal system for light-driven reversible reduction of chemical species that, on their own, do not respond to light. This is achieved by the unique combination of photo-sensitive plasmonic aggregates and temperature-responsive inorganic species generating radicals that can be finally accepted and stabilised by non-photo-responsive organic molecules. In this system Au nanoparticles (NPs) self-assembled via sub-nm precise molecular spacers (cucurbit[n]urils) interact strongly with visible light to locally accelerate the decomposition of dithionite species (S(2)O(4)(2–)) close to the NP interfaces. This light-driven process leads to the generation of inorganic radicals whose electrons can then be reversibly picked up by small organic acceptors, such as the methyl viologen molecules (MV(2+)) used here. During light-triggered plasmon- and heat-assisted generation of radicals, the S(2)O(4)(2–) species work as a chemical ‘fuel’ linking photo-induced processes at the NP interfaces with redox chemistry in the surrounding water environment. By incorporating MV(2+) as a Raman-active reporter molecule, the resulting optically-controlled redox processes can be followed in real-time.

Quantitative applications of surface enhanced Raman spectroscopy (SERS) often rely on surface partition layers grafted to SERS substrates to collect and trap solvated analytes that would not otherwise adsorb onto metals. Such binding layers drastically broaden the scope of analytes that can be probed. However, excess binding sites introduced by this partition layer also trap analytes outside the plasmonic †hot-spots'. We show that by eliminating these binding sites, limits of detection (LODs) can effectively be lowered by more than an order of magnitude. We highlight the effectiveness of this approach by demonstrating quantitative detection of controlled drugs down to sub-nanomolar concentrations in aqueous media. Such LODs are low enough to screen, for example, urine at clinically relevant levels. These findings provide unique insights into the binding behavior of analytes, which are essential when designing high performance SERS substrates.

In this paper, we propose a frequency ramp generator for very fast network analysis in the frequency range from 0.4 GHz to 6 GHz. Highly linear analog frequency ramps are synthesized at high frequencies (8.4 GHz to 14 GHz), using a wideband voltage-controlled oscillator (VCO) stabilized by a fractional-N phase locked loop (PLL), and are mixed down to the desired frequency band using a second PLL-stabilized VCO at a constant frequency of 14.4 GHz. In order to achieve a wideband PLL-stabilization, the loop gain variation is reduced by inserting a voltage dependent damping network between the loop filter and the wideband VCO. This leads to an almost constant transient and phase noise performance in the complete frequency range. The overall phase noise at the output of the ramp generator is below −99dBc/Hz at offset frequencies above 10 kHz. The proposed ramp generator allows for fast and accurate network analysis which is demonstrated by means of measurements with a homodyne prototype system. The sweep time of the system (0.5 ms) is significantly shorter compared to previously published ramp based systems and commercial network analyzers, and allows monitoring of rapidly changing measurement scenarios.

In this paper, we present a multisection directional coupler providing a tight coupling of 10 dB with an equal ripple of 0.8 dB across the frequency range of 360 MHz to 5.8 GHz, which corresponds to a relative bandwidth of 177 %. To maximize coupling and reduce dispersion effects, the coupler uses broadside coupled striplines in a multilayer setup. The four ports of the coupler are connected via standard SMA-connectors through a stripline to microstrip transition that does not make use of vias. The design process is verified by EM simulations and measurements performed on a prototype.

Trapping light to atomic scales enables yoctoliter spectroscopy, unveiling individual molecular deprotonation events., Understanding single-molecule chemical dynamics of surface ligands is of critical importance to reveal their individual pathways and, hence, roles in catalysis, which ensemble measurements cannot see. Here, we use a cascaded nano-optics approach that provides sufficient enhancement to enable direct tracking of chemical trajectories of single surface-bound molecules via vibrational spectroscopy. Atomic protrusions are laser-induced within plasmonic nanojunctions to concentrate light to atomic length scales, optically isolating individual molecules. By stabilizing these atomic sites, we unveil single-molecule deprotonation and binding dynamics under ambient conditions. High-speed field-enhanced spectroscopy allows us to monitor chemical switching of a single carboxylic group between three discrete states. Combining this with theoretical calculation identifies reversible proton transfer dynamics (yielding effective single-molecule pH) and switching between molecule-metal coordination states, where the exact chemical pathway depends on the intitial protonation state. These findings open new domains to explore interfacial single-molecule mechanisms and optical manipulation of their reaction pathways.