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Speedy, point-of-need detection and monitoring of small-molecule metabolites are vital across diverse applications ranging from biomedicine to agri-food and environmental surveillance. Nanomaterial-based sensor (nanosensor) platforms are rapidly emerging as excellent candidates for versatile and ultrasensitive detection owing to their highly configurable optical, electrical and electrochemical properties, fast readout, as well as portability and ease of use. To translate nanosensor technologies for real-world applications, key challenges to overcome include ultralow analyte concentration down to ppb or nM levels, complex sample matrices with numerous interfering species, difficulty in differentiating isomers and structural analogues, as well as complex, multidimensional datasets of high sample variability. In this Perspective, we focus on contemporary and emerging strategies to address the aforementioned challenges and enhance nanosensor detection performance in terms of sensitivity, selectivity and multiplexing capability. We outline 3 main concepts: (1) customization of designer nanosensor platform configurations via chemical- and physical-based modification strategies, (2) development of hybrid techniques including multimodal and hyphenated techniques, and (3) synergistic use of machine learning such as clustering, classification and regression algorithms for data exploration and predictions. These concepts can be further integrated as multifaceted strategies to further boost nanosensor performances. Finally, we present a critical outlook that explores future opportunities toward the design of next-generation nanosensor platforms for rapid, point-of-need detection of various smallmolecule metabolites. Agency for Science, Technology and Research (A*STAR) Nanyang Technological University National Medical Research Council (NMRC) Published version This research is supported by the Singapore National Medical Research Council COVID-19 Grant (MOH-000584) and A*STAR AME Individual Research Grant (A20E5c0082). S. X. L. and L. B. T. N. acknowledge Nanyang President's Graduate Scholarship support from Nanyang Technological University, Singapore.

Because of the versatility of plasmonic nanoparticles, there have been major improvements in tailoring the plasmonic effects for a plethora of applications. However, a major bottleneck of plasmonic...

Gas-phase surface-enhanced Raman scattering (SERS) remains challenging due to poor analyte affinity to SERS substrates. The reported use of capturing probes suffers from concurrent inconsistent signals and long response time due to the formation of multiple potential probe-analyte interaction orientations. Here, we demonstrate the use of multiple non-covalent interactions for ring complexation to boost the affinity of small gas molecules, SO2 and NO2 , to our SERS platform, achieving rapid capture and multiplex detection down to 100 ppm. Experimental and in-silico studies affirm stable ring complex formation, and kinetic investigations reveal a 4-fold faster response time compared to probes without stable ring complexation capability. By synergizing spectral concatenation and support vector machine regression, we achieve 91.7 % accuracy for multiplex quantification of SO2 and NO2 in excess CO2 , mimicking real-life exhausts. Our platform shows immense potential for on-site exhaust and air quality surveillance. Agency for Science, Technology and Research (A*STAR) National Research Foundation (NRF) Submitted/Accepted version This research is supported by Singapore National Research Foundation Central Gap Fund (NRF2020NRF-CG001-010) and A*STAR AME Individual Research Grant (A20E5c0082).

The electrochemical nitrogen reduction reaction (NRR) offers a sustainable solution towards ammonia production but suffers poor reaction performance owing to preferential catalyst–H formation and the consequential hydrogen evolution reaction (HER). Now, the Pt/Au electrocatalyst d-band structure is electronically modified using zeolitic imidazole framework (ZIF) to achieve a Faradaic efficiency (FE) of > 44% with high ammonia yield rate of > 161 mgmgcat @1 h @1 under ambient conditions. The strategy lowers electrocatalyst d-band position to weaken H adsorption and concurrently creates electron-deficient sites to kinetically drive NRR by promoting catalyst–N2 interaction. The ZIF coating on the electrocatalyst doubles as a hydrophobic layer to suppress HER, further improving FE by > 44-fold compared to without ZIF (ca. 1%). The Pt/Au-NZIF interaction is key to enable strong N2 adsorption over H atom. Ministry of Education (MOE) Nanyang Technological University This research is supported by the Ministry of Education, Singapore, under Tier 1 (RG11/18) and Tier 2 (MOE2016-T2- 1-043) grants, and Max Planck Institute-Nanyang Technological University Joint Lab. H.Y.F.S., C.S.L.K. and G.C.P.-Q. thank scholarship support from Nanyang Technological University, Singapore. J.R.T.C. and J.Y.P. thank CN Yang scholarship. H.K.L. thanks the International Postdoctoral Scholarship support from Nanyang Technological University, Singapore, and Singapore Ministry of Education. We thank Mr. Poh Chong Lim, A*STAR, for XRD analysis.

Probing changes of noncovalent interactions is crucial to study the binding efficiencies and strengths of (bio)molecular complexes. While surface-enhanced Raman scattering (SERS) offers unique molecular fingerprints to examine such interactions in situ, current platforms are only able to recognize hydrogen bonds because of their reliance on manual spectral identification. Here, we differentiate multiple intermolecular interactions between two interacting species by synergizing plasmonic liquid marble-based SERS platforms, chemometrics, and density functional theory. We demonstrate that characteristic 3-mercaptobenzoic acid (probe) Raman signals have distinct peak shifts upon hydrogen bonding and ionic interactions with tert-butylamine, a model interacting species. Notably, we further quantify the contributions from each noncovalent interaction coexisting in different proportions. As a proof-of-concept, we detect and categorize biologically important nucleotide bases based on molecule-specific interactions. This will potentially be useful to study how subtle changes in biomolecular interactions affect their structural and binding properties.

Here we design an interface between a metal nanoparticle (NP) and a metal-organic framework (MOF) to activate an inert CO₂ carboxylation reaction and in situ monitor its unconventional regioselectivity at the molecular level. Using a Kolbe-Schmitt reaction as model, our strategy exploits the NP@MOF interface to create a pseudo high-pressure CO₂ microenvironment over the phenolic substrate to drive its direct C-H carboxylation at ambient conditions. Conversely, Kolbe-Schmitt reactions usually demand high reaction temperature (>125 °C) and pressure (>80 atm). Notably, we observe an unprecedented CO₂ meta-carboxylation of an arene that was previously deemed impossible in traditional Kolbe-Schmitt reactions. While the phenolic substrate in this study is fixed at the NP@MOF interface to facilitate spectroscopic investigations, free reactants could be activated the same way by the local pressurized CO₂ microenvironment. These valuable insights create enormous opportunities in diverse applications including synthetic chemistry, gas valorization, and greenhouse gas remediation. Ministry of Education (MOE) Nanyang Technological University X.Y.L. thanks the Singapore Ministry of Education for Tier 1 (RG11/18) and Tier 2 (MOE2016-T2-1-043) grants, and Max Planck Institute-Nanyang Technological University Joint Lab. C.-K.T. appreciates the funding support from Boston College and the NSF (CHE 1566445). H.K.L. thanks the Nanyang Technological University and the Ministry of Education, Singapore for the International Postdoctoral Scholarship. C.S.L.K. and G.C.P-Q. acknowledge scholarship support from Nanyang Technological University, Singapore.

Harnessing large hotspot volumes is key for enhanced gas-phase surface-enhanced Raman scattering (SERS) sensing. Herein, we introduce versatile, air-stable 3D 'Plasmonic bubbles' with bi-directional sensing capabilities. Our Plasmonic bubbles are robust, afford strong and homogenous SERS signals, and can swiftly detect both encapsulated and surrounding 4-methylbenzenethiol vapors. Published version

Disease X is a hypothetical unknown disease that has the potential to cause an epidemic or pandemic outbreak in the future. Nanosensors are attractive portable devices that can swiftly screen disease biomarkers on site, reducing the reliance on laboratory-based analyses. However, conventional data analytics limit the progress of nanosensor research. In this Perspective, we highlight the integral role of machine learning (ML) algorithms in advancing nanosensing strategies toward Disease X detection. We first summarize recent progress in utilizing ML algorithms for the smart design and fabrication of custom nanosensor platforms as well as realizing rapid on-site prediction of infection statuses. Subsequently, we discuss promising prospects in further harnessing the potential of ML algorithms in other aspects of nanosensor development and biomarker detection. Nanyang Technological University Submitted/Accepted version S.X.L. and N.B.T.L. acknowledge Nanyang President’s Graduate Scholarship support from Nanyang Technological University, Singapore.

Population-wide surveillance of COVID-19 requires tests to be quick and accurate to minimize community transmissions. The detection of breath volatile organic compounds presents a promising option for COVID-19 surveillance but is currently limited by bulky instrumentation and inflexible analysis protocol. Here, we design a hand-held surface-enhanced Raman scattering-based breathalyzer to identify COVID-19 infected individuals in under 5 min, achieving >95% sensitivity and specificity across 501 participants regardless of their displayed symptoms. Our SERS-based breathalyzer harnesses key variations in vibrational fingerprints arising from interactions between breath metabolites and multiple molecular receptors to establish a robust partial least-squares discriminant analysis model for high throughput classifications. Crucially, spectral regions influencing classification show strong corroboration with reported potential COVID-19 breath biomarkers, both through experiment and in silico. Our strategy strives to spur the development of next-generation, noninvasive human breath diagnostic toolkits tailored for mass screening purposes. Agency for Science, Technology and Research (A*STAR) Nanyang Technological University National Medical Research Council (NMRC) Submitted/Accepted version This research is supported by National Medical Research Council, Singapore under COVID-19 Research Fund (MOH-COVID19RF-0007 and MOH-COVID19RF-0012), A*STAR Singapore, AME Individual Research Grant (A20E5c0082) and Max Planck Institute-Nanyang Technological University Joint Lab. S.X.L and L.B.T.N. acknowledge Nanyang Presidential scholarship support from Nanyang Technological University, Singapore.

Nanoplasmonic materials play a critical role in surface-enhanced Raman scattering (SERS) because they dictate the electromagnetic (EM) field confinement at the nanoscale and hence the SERS enhancements. These materials can be further engineered to maximize SERS sensitivity, even for analytes with no specific affinity to plasmonic surfaces. In this chapter, we discuss the various types and design strategies of nanoplasmonic materials and their platforms which are employed to boost SERS sensing performance. Four major approaches are outlined: (1) zero-dimensional to three-dimensional hotspot engineering for intense EM field confinement and interparticle plasmonic coupling, (2) physical confinement of liquid analytes on superhydrophobic SERS-active substrates, (3) concentration of gaseous analytes over hotspots using highly porous materials, and (4) development of unconventional nanoplasmonic materials including bimetallic configurations and hybrid systems. Finally, we finish this chapter by discussing current challenges in this research area.

Electrochemical nitrogen reduction reaction (NRR) offers sustainable ammonia production but suffers from poor performance owing to favorable water electrolysis. Recent designs achieve better efficiency by eradicating water but do not leverage on water as a readily available NRR proton source. Herein, we design a hydrophobic oleylamine-functionalized zeolitic-imidazolate framework coated over the electrocatalyst to achieve >18% NRR efficiency in the presence of water, an approximately fourfold boost compared to that without water. Our strategy kinetically regulates water availability at the electrocatalyst surface, suppresses direct water adsorption/electrolysis, and promotes preferential nitrogen adsorption to achieve water-assisted NRR. Conversely, control systems without hydrophobic modification experience a drastic decrease in efficiencies (

Stand-off Raman spectroscopy combines the advantages of both Raman spectroscopy and remote detection to retrieve molecular vibrational fingerprints of chemicals at inaccessible sites. However, it is currently restricted to the detection of pure solids and liquids and not widely applicable for dispersed molecules in air. Herein, we realize real-time stand-off SERS spectroscopy for remote and multiplex detection of atmospheric airborne species by integrating a long-range optic system with a 3D analyte-sorbing metal–organic framework (MOF)-integrated SERS platform. Formed via the self-assembly of Ag@MOF core–shell nanoparticles, our 3D plasmonic architecture exhibits micrometer thick SERS hotspot to allow active sorption and rapid detection of aerosols, gas, and volatile organic compounds down to parts-per-billion levels, notably at a distance up to 10 m apart. The platform is highly sensitive to changes in atmospheric content, as demonstrated in the temporal monitoring of gaseous CO2 in several cycles. Importantly, we demonstrate the remote and multiplex quantification of polycyclic aromatic hydrocarbon mixtures in real time under outdoor daylight. By overcoming core challenges in current remote Raman spectroscopy, our strategy creates an opportunity in the long-distance and sensitive monitoring of air/gaseous environment at the molecular level, which is especially important in environmental conservation, disaster prevention, and homeland defense. Ministry of Education (MOE) Nanyang Technological University Accepted version X.Y.L. thanks the Singapore Ministry of Education Tier 1 (RG11/18) and Tier 2 (MOE2016-T2-1-043) grants. G.C.P.-Q. and C.S.L.K. are thankful for Nanyang President’s Graduate Scholarships. N.Y. is thankful for the CN Yang scholarship. T.L. acknowledges the funding support from the National Natural Science Foundation of China (51433001).

Integrating machine learning with surface-enhanced Raman scattering (SERS) accelerates the development of practical sensing devices. Such integration, in combination with direct detection or indirect analyte capturing strategies, is key to achieving high predictive accuracies even in complex matrices. However, in-depth understanding of spectral variations arising from specific chemical interactions is essential to prevent model overfit. Herein, we design a machine-learning-driven "SERS taster" to simultaneously harness useful vibrational information from multiple receptors for enhanced multiplex profiling of five wine flavor molecules at parts-per-million levels. Our receptors employ numerous noncovalent interactions to capture chemical functionalities within flavor molecules. By strategically combining all receptor-flavor SERS spectra, we construct comprehensive "SERS superprofiles" for predictive analytics using chemometrics. We elucidate crucial molecular-level interactions in flavor identification and further demonstrate the differentiation of primary, secondary, and tertiary alcohol functionalities. Our SERS taster also achieves perfect accuracies in multiplex flavor quantification in an artificial wine matrix. Agency for Science, Technology and Research (A*STAR) Ministry of Health (MOH) This research is supported by the A*STAR AME Individual Research Grant (A20E5c0082), NMRC Grant (MOH-000503), and Max Planck Institute-Nanyang Technological University Joint Lab.

Photothermal materials are crucial for diverse heating applications, but it remains challenging to achieve high energy conversion efficiency due to the difficulty to concurrently improve light absorbance and suppress heat loss. Herein, a zeolitic imidazolate framework-isolated graphene (G@ZIF) nanohybrid is demonstrated that utilizes ultrathin, heat-insulating ZIF layers, and G@ZIF interfacial nanocavity to synergistically intensify light absorbance and heat localization. Under artificial sunlight illumination (≈1 kW m−2), the G@ZIF film attains a maximum temperature of 120 °C in an open environment with a 98% solar-to-thermal conversion efficiency. Importantly, the porous ZIF layer allows small molecules/media to enter and access the embedded hot graphene surface for targeted heat transfer in practical applications. As a proof-of-concept, the G@ZIF-based steam generator realizes 96% energy conversion from light to vapor with near-perfect desalination and water purification efficiencies (>99.9%). This design is generic and can be extended to other photothermal systems for advanced solar-thermal applications, including catalysis, water treatments, sterilization, and mechanical actuation. Ministry of Education (MOE) This research was supported by the Ministry of Education, Singapore, under Tier 1 (RG11/18 and RG97/19) and Tier 2 (MOE2016-T2-1-043) grants.

Surface-enhanced Raman scattering (SERS) is a molecule-specific spectroscopic technique with diverse applications in (bio)chemistry, clinical diagnosis and toxin sensing. While hotspot engineering has expedited SERS development, it is still challenging to detect molecules with no specific affinity to plasmonic surfaces. With the aim of improving detection performances, we venture beyond hotspot engineering in this tutorial review and focus on emerging material design strategies to capture and confine analytes near SERS-active surfaces as well as various promising hybrid SERS platforms. We outline five major approaches to enhance SERS performance: (1) enlarging Raman scattering cross-sections of non-resonant molecules via chemical coupling reactions; (2) targeted chemical capturing of analytes through surface-grafted agents to localize them on plasmonic surfaces; (3) physically confining liquid analytes on non-wetting SERS-active surfaces and (4) confining gaseous analytes using porous materials over SERS hotspots; (5) synergizing conventional metal-based SERS platforms with functional materials such as graphene, semiconducting materials, and piezoelectric polymers. These approaches can be integrated with engineered hotspots as a multifaceted strategy to further boost SERS sensitivities that are unachievable using hotspot engineering alone. Finally, we highlight current challenges in this research area and suggest new research directions towards efficient SERS designs critical for real-world applications. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Nanyang Technological University Accepted version X. Y. L. thanks the financial support from Singapore Ministry of Education, Tier 1 (RG21/16) and Tier 2 (MOE2016-T2-1-043) grants. C. L. L. acknowledges the A*STAR Graduate Scholarship from A*STAR, Singapore. C. S. L. K. and G. C. P.-Q. acknowledge support from Nanyang Presidential Graduate Scholarship from Nanyang Technological University. Q. A. thanks the funding support from NSFC (21303169, 21673209, 51572246), the Fundamental Research Funds for the Central Universities (2652015295), and Beijing Nova Program (Z141103001814064).

Gas‐liquid reactions form the basis of our everyday lives, yet they still suffer poor reaction efficiency and are difficult to monitor in situ, especially at ambient conditions. Herein, we drive an inert gas‐liquid reaction between aniline and CO2 at 1 atm and 298 K by selectively concentrating these immiscible reactants at the interface between metal‐organic framework and solid nanoparticles (solid@MOF). Real‐time reaction SERS monitoring and simulation investigations affirm the formation of phenylcarbamic acid, which was previously undetectable because they are unstable for post‐reaction treatments. The solid@MOF ensemble gives rise to a >28‐fold improvement to reaction efficiency as compared to ZIF‐only and solid‐only platforms, emphasizing that the interfacial nanocavities in solid@MOF are the key to enhance gas‐liquid reaction. Our strategy can be integrated with other functional materials, hence opens up new opportunities for ambient‐operated gas‐liquid applications. MOE (Min. of Education, S’pore) Accepted version

The application of aluminum (Al)-based nanostructures for visible-range plasmonics, especially for surface-enhanced Raman scattering (SERS), currently suffers from inconsistent local electromagnetic field distributions and/or inhomogeneous distribution of probe molecules. Herein, we lithographically fabricate structurally uniform Al nanostructures which enable homogeneous adsorption of various probe molecules. Individual Al nanostructures exhibit strong local electromagnetic field enhancements, in turn leading to intense SERS activity. The average SERS enhancement factor (EF) for individual nanostructures exceeds 104 for non-resonant probe molecules in the visible spectrum. These Al nanostructures also retain more than 70% of their original SERS intensities after one-month storage, displaying superb stability under ambient conditions. We further achieve tunable polarization-dependent SERS responses using anisotropic Al nanostructures, facilitating the design of sophisticated SERS-based security labels. Our micron-sized security label comprises two-tier security features, including a machine-readable hybrid quick-response (QR) code overlaid with a set of ciphertexts. Our work demonstrates the versatility of Al-based structures in low-cost modern chemical nano-analytics and forgery protection. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

Nanoporous gold (NPG) promises efficient light-to-heat transformation, yet suffers limited photothermal conversion efficiency owing to the difficulty in controlling their morphology for direct modulation of thermo-plasmonic properties. Herein, we showcase a series of shape-controlled NPG nanoparticles with distinct bowl- (NPG-B), tube- (NPG-T) and plate-like (NPG-P) structures for quantitative temperature regulation up to 140 oC in < 1 s using laser irradiation. Notably, NPG-B exhibits a highest photothermal efficiency of 68% which is >12 and 39 percentage points better than other NPG shapes (NPG-T, 56%; NPG-P, 49%) and Au nanoparticles (29%), respectively. We attribute NPG-B’s superior photothermal performance to its >13% enhanced light absorption cross section compared to other Au nanostructures. We further realize an ultrasensitive heat-mediated light-to-mechanical “kill switch” by integrating NPG-B with a heat-responsive shape-memory polymer (SMP/NPG-B). This SMP/NPG-B hybrid is analogous to a photo-triggered mechanical arm, and can be activated swiftly in

Particle-assembled fluid-fluid interfaces give rise to soft substrate-less platforms with wide-ranging applications, including remote and on-demand manipulation, optical modulation, catalysis, and multiphase and multiplex sensing, as well as in situ reaction kinetics elucidation. Notably, these soft platforms are easy to fabricate and can exhibit long-range order, both of which are challenging to achieve using traditional solid-based substrates. In this perspective, we provide an overview of the latest research in the fabrication and applications of these soft platforms. We begin with a brief discussion on the formation mechanism of two- and three-dimensional substrate-less platforms, followed by highlighting the unique properties of these platforms. We also discuss the application of these particle-assembled interfaces to three specific research areas, including dynamic tuning of optical properties, multiplex molecular sensing, and small-volume reaction modulation and kinetics monitoring. We end our perspective with an outlook on the promising research frontiers that can be achieved using these soft substrate-less platforms. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) Accepted version X.Y.L. is grateful for the financial support from National Research Foundation, Singapore (NRF-NRFF2012-04), Singapore Ministry of Education, Tier 1 (RG21/16) and Tier 2 (MOE2016-T2-1-043) grants. H.K.L. appreciates the A*STAR Graduate Scholarship support from A*STAR, Singapore. G.C.P.-Q. and C.S.L.K acknowledge support from Nanyang Presidential Graduate Scholarship from Nanyang Technological University.

Liquid marbles are emergent microreactors owing to their isolated environment and the flexibility of materials used. Plasmonic liquid marbles (PLMs) are demonstrated as the smallest spectroelectrochemical microliter‐scale reactor for concurrent spectro‐ and electrochemical analyses. The three‐dimensional Ag shell of PLMs are exploited as a bifunctional surface‐enhanced Raman scattering (SERS) platform and working electrode for redox process modulation. The combination of SERS and electrochemistry (EC) capabilities enables in situ molecular read‐out of transient electrochemical species, and elucidate the potential‐dependent and multi‐step reaction dynamics. The 3D configuration of our PLM‐based EC‐SERS system exhibits 2‐fold and 10‐fold superior electrochemical and SERS performance than conventional 2D platforms. The rich molecular‐level electrochemical insights and excellent EC‐SERS capabilities offered by our 3D spectroelectrochemical system are pertinent in charge transfer processes. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) Accepted version X.Y.L. thanks the financial support from National Research Foundation, Singapore (NRF-NRFF2012-04), Singapore Ministry of Education, Tier 1(RG21/16) and Tier (MOE2016-T2-1-043 (S)) grants, and Nanyang Technological University.C.S.L.K.,G.C.P.-Q., and M.R.L. acknowledge support from Nanyang Presidential Graduate Scholarship. H.K.L. thanks the graduate scholarship from A*STAR, Singapore

Two-photon lithography (TPL) is an emerging approach to fabricate complex multifunctional micro/nanostructures. This is because TPL can easily develop various 2D and 3D structures on a variety of surfaces, and there has been a rapidly expanding pool of processable photoresists to create different materials. However, challenges in developing two-photon processable photoresists currently impede progress in TPL. In this review, we critically discuss the importance of photoresist formulation in TPL. We begin by evaluating the commercial photoresists to design micro/nanostructures for promising applications in anti-counterfeiting, superomniphobicity, and micromachines with movable parts. Next, we discuss emerging hydrogel/organogel photoresists, focusing on customizing photoresist formulations to fabricate reconfigurable structures that can respond to changes in local pH, solvent, and temperature. We also review the development of metal salt-based photoresists for direct metal writing, whereby various formulations have been developed to enable applications in online sensing, catalysis, and electronics. Finally, we provide a critical outlook and highlight various outstanding challenges in formulating processable photoresists for TPL. Ministry of Education (MOE) Nanyang Technological University Accepted version X.Y.L. thanks Singapore Ministry of Education, Tier 1 (RG11/18) and Tier 2 (MOE2016-T2-1-043) grants, and Max PlanckInstitute−Nanyang Technological University Joint Lab for the financial support. C S.L.K., G.C.P.-Q., and S.X.L. thank the Nanyang President’s Graduate Scholarships.

Harvesting solar energy via pyroelectric technology is highly desired because it is a sustainable and clean solution to yield electricity. However, the low power output (below 5mW/m2) and bulky device design of traditional devices hinder their practical applicability. Herein, we demonstrate a highly efficient sunlight-triggered pyroelectric nanogenerator (S-PENG) design that achieves unprecedented performance of 21.3 mW/m2 under one sun irradiation. This is achieved via enhancing both the solar harvesting properties and the pyroelectric efficiencies rationally by employing polyethyleneimine (PEI) chemically modified reduced graphene oxide (rGO-PEI), and a pyroelectric layer based on a polarized polyvinylidene fluoride (PVDF) film respectively. Moreover, we demonstrate its practical application by adapting the S-PENG into a wearable outdoor bracelet. Using a facile hand-waving motion to replace the traditional bulky sunlight-alternating device, our compact bracelet can charge a fitness tracker after only 1 h of outdoor exposure, demonstrating its capability to be seamlessly integrated with outdoor sports activities. We expect our rational design to promote the development of S-PENG and pave the way for the utilization of sunlight as a viable on-the-go electrical source.

Hybrid materials of earth abundant transition metal dichalcogenides and noble metal nanoparticles, such as molybdenum sulfide (MoSx) and gold nanoparticles, exhibit synergistic effects that can enhance electrocatalytic reactions. However, most current hybrid MoSx-gold synthesis requires an energy intensive heat source of >500 °C or chemical plating to achieve deposition of MoSx on the gold surface. Herein, we demonstrate the direct overgrowth of MoSx over colloidal nanoporous gold (NPG), conducted feasibly under ambient conditions, to form hybrid particles with enhanced electrocatalytic performance toward hydrogen evolution reaction. Our strategy exploits the localized surface plasmon resonance-mediated photothermal heating of NPG to achieve >230 °C surface temperature, which induces the decomposition of the (NH4)2MoS4 precursor and direct overgrowth of MoSx over NPG. By tuning the concentration ratio between the precursor and NPG, the amount of MoSx particles deposited can be systematically controlled from 0.5% to 2% of the Mo/(Au + Mo) ratio. Importantly, we find that the hybrid particles exhibit higher bridging and an apical S to terminal S atomic ratio than pure molybdenum sulfide, which gives rise to their enhanced electrocatalytic performance for hydrogen evolution reaction. We demonstrate that hybrid MoSx-NPG exhibits >30 mV lower onset potential and a 1.7-fold lower Tafel slope as compared to pure MoSx. Our methodology provides an energy- and cost-efficient synthesis pathway, which can be extended to the synthesis of various functional hybrid structures with unique properties for catalysis and sensing applications. Ministry of Education (MOE) Nanyang Technological University Published version X. Y. Ling acknowledges Singapore Ministry of Education, Tier 1 (No. RG11/18) and Tier 2 (No. MOE2016-T2-1-043) grants. G. C. Phan-Quang, C. S. L. Koh, and S. X. Leong acknowledge the Nanyang President’s Graduate Scholarship.

Surface-enhanced Raman scattering (SERS) is a molecular-specific spectroscopic technique that provides up to 1010-fold enhancement of signature Raman fingerprints using nanometer-scale 0D to 2D platforms. Over the past decades, 3D SERS platforms with additional plasmonic materials in the z-axis have been fabricated at sub-micrometer to centimeter scale, achieving higher hotspot density in all x, y, and z spatial directions and higher tolerance to laser misalignment. Moreover, the flexibility to construct platforms in arbitrary sizes and 3D shapes creates attractive applications besides traditional SERS sensing. In this Account, we introduce our library of substrate-based and substrate-less 3D plasmonic platforms, with an emphasis on their non-sensing applications as microlaboratories and data storage labels. We aim to provide a scientific synopsis on these high-potential yet currently overlooked applications of SERS and ignite new scientific discoveries and technology development in 3D SERS platforms to tackle real-world issues. One highlight of our substrate-based SERS platforms is multilayered platforms built from micrometer-thick assemblies of plasmonic particles, which can achieve up to 1011 enhancement factor. As an alternative, constructing 3D hotspots on non-plasmonic supports significantly reduces waste of plasmonic materials while allowing high flexibility in structural design. We then introduce our emerging substrate-less plasmonic capsules including liquid marbles and colloidosomes, which we further incorporate the latter within an aerosol to form centimeter-scale SERS-active plasmonic cloud, the world's largest 3D SERS platform to date. We then discuss the various emerging applications arising only from these 3D platforms, in the fields of sensing, microreactions, and data storage. An important novel sensing application is the stand-off detection of airborne analytes that are several meters away, made feasible with aerosolized plasmonic clouds. We also describe plasmonic capsules as excellent miniature lab-in-droplets that can simultaneously provide in situ monitoring at the molecular level during reaction, owing to their ultrasensitive 3D plasmonic shells. We highlight the emergence of 3D SERS-based data storage platforms with 10-100-fold higher storage density than 2D platforms, featuring a new approach in the development of level 3 security (L3S) anti-counterfeiting labels. Ultimately, we recognize that 3D SERS research can only be developed further when its sensing capabilities are concurrently strengthened. With this vision, we foresee the creation of highly applicable 3D SERS platforms that excel in both sensing and non-sensing areas, providing modern solutions in the ongoing Fourth Industrial Revolution. Ministry of Education (MOE) Accepted version This work was funded by Singapore Ministry of Education, Tier 1 (RG11/18) and Tier 2 (MOE2016-T2-1-043) grants, and Max Planck Institute-Nanyang Technological University Joint Lab, the Spanish Ministerio de Economia y Competitividad (CTQ2017-88648R and RYC-2015-19107), the Generalitat de Cataluña (2017SGR883), the Universitat Rovira i Virgili (2017PFR-URV-B2-02), and the Universitat Rovira i Virgili and Banco Santander (2017EXIT-08).

Timely detection of toxic vapor is vital for safeguarding people's lives. Herein, we design a plasmonic nose based on a zeolitic imidazolate framework (ZIF)-encapsulated Ag nanocube array for ultratrace recognition of VOC vapor. The plasmonic nose enables in situ adsorption kinetics and recognition of various VOCs at ppm levels, eliminating false positives. Our approach provides a paradigm shift to next-generation, effective and specific gas sensors. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

Molecular‐level airborne sensing is critical for early prevention of disasters, diseases, and terrorism. Currently, most 2D surface‐enhanced Raman spectroscopy (SERS) substrates used for air sensing have only one functional surface and exhibit poor SERS‐active depth. “Aerosolized plasmonic colloidosomes” (APCs) are introduced as airborne plasmonic hotspots for direct in‐air SERS measurements. APCs function as a macroscale 3D and omnidirectional plasmonic cloud that receives laser irradiation and emits signals in all directions. Importantly, it brings about an effective plasmonic hotspot in a length scale of approximately 2.3 cm, which affords 100‐fold higher tolerance to laser misalignment along the z‐axis compared with 2D SERS substrates. APCs exhibit an extraordinary omnidirectional property and demonstrate consistent SERS performance that is independent of the laser and analyte introductory pathway. Furthermore, the first in‐air SERS detection is demonstrated in stand‐off conditions at a distance of 200 cm, highlighting the applicability of 3D omnidirectional plasmonic clouds for remote airborne sensing in threatening or inaccessible areas. MOE (Min. of Education, S’pore) Accepted version

Originally unfavored nitrogen-to-ammonia electroconversion is now preferred over competing reaction using reticular chemistry., Electrochemical nitrogen-to-ammonia fixation is emerging as a sustainable strategy to tackle the hydrogen- and energy-intensive operations by Haber-Bosch process for ammonia production. However, current electrochemical nitrogen reduction reaction (NRR) progress is impeded by overwhelming competition from the hydrogen evolution reaction (HER) across all traditional NRR catalysts and the requirement for elevated temperature/pressure. We achieve both excellent NRR selectivity (~90%) and a significant boost to Faradic efficiency by 10 percentage points even at ambient operations by coating a superhydrophobic metal-organic framework (MOF) layer over the NRR electrocatalyst. Our reticular chemistry approach exploits MOF’s water-repelling and molecular-concentrating effects to overcome HER-imposed bottlenecks, uncovering the unprecedented electrochemical features of NRR critical for future theoretical studies. By favoring the originally unfavored NRR, we envisage our electrocatalytic design as a starting point for high-performance nitrogen-to-ammonia electroconversion directly from water vapor–abundant air to address increasing global demand of ammonia in (bio)chemical and energy industries.

Miniaturizing the continuous multistep operations of a factory into a microchemical plant offers a safe and cost-effective approach to promote high-throughput screening in drug development and enforcement of industrial/environmental safety. While particle-assembled microdroplets in the form of liquid marble are ideal as microchemical plant, these platforms are mainly restricted to single-step reactions and limited to ex situ reaction monitoring. Herein, we utilize plasmonic liquid marble (PLM), formed by encapsulating liquid droplet with Ag nanocubes, to address these issues and demonstrate it as an ideal microchemical plant to conduct reaction-and-detection sequences on-demand in a nondisruptive manner. Utilizing a two-step azo-dye formation as our model reaction, our microchemical plant allows rapid and efficient diazotization of nitroaniline to form diazonium nitrobenzene, followed by the azo coupling of this intermediate with target aromatic compound to yield azo-dye. These molecular events are tracked in situ via SERS measurement through the plasmonic shell and further verified with in silico investigation. Furthermore, we apply our microchemical plant for ultrasensitive SERS detection and quantification of bisphenol A (BPA) with detection limit down to 10 amol, which is 50 000-fold lower than the BPA safety limit. Together with the protections offered by plasmonic shell against external environments, these collective advantages empower PLM as a multifunctional microchemical plant to facilitate small-volume testing and optimization of processes relevant in industrial and research contexts. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Nanyang Technological University Accepted version X.Y.L thanks the support from Singapore Ministry of Education, Tier 1 (RG21/16) and Tier 2 (MOE2016-T2-1-043). C.S.L.K. thanks Nanyang Technological University, Nanyang Presidential Graduate Scholarship. H.K.L. appreciates the A*STAR Graduate Scholarship support from A*STAR, Singapore.

We demonstrate a molecular-level observation of driving CO

By designing a triboelectrically active and dielectric composite substrate, we introduce a new approach to obtain the electric-field boosted surface-enhanced Raman spectroscopy detection (E-SERS), which strengthens SERS signals of conventional noble metal nanoparticle-based platforms by 3-fold. The substrate is composed of Au–Ag binary metal nanostructures deposited on a composite dielectric polymeric film. The triboelectricity is easily generated by simply rubbing the SERS substrate using a piece of copper foil. The electricity then enhances the hotspot intensities in the noble metal structures, and boosts SERS signals obtained therein. Using these substrates, facile detections of various molecules even in corrosive (oxidizing) solutions are achieved with enhanced sensitivities and high reproducibility. This rational design for E-SERS relatively simplifies the conventional setup requirements and the operational complexity of conventional E-SERS detections. We expect our triboelectrically-enabled E-SERS concept can promote practical on-site E-SERS detections.

We demonstrate a one-step precise direct metal writing of well-defined and densely packed gold nanoparticle (AuNP) patterns with tunable physical and optical properties. We achieve this by using two-photon lithography on a Au precursor comprising poly(vinylpyrrolidone) (PVP) and ethylene glycol (EG), where EG promotes higher reduction rates of Au(III) salt via polyol reduction. Hence, clusters of monodisperse AuNP are generated along raster scanning of the laser, forming high-particle-density, well-defined structures. By varying the PVP concentration, we tune the AuNP size from 27.3 to 65.0 nm and the density from 172 to 965 particles/μm2, corresponding to a surface roughness of 12.9 to 67.1 nm, which is important for surface-based applications such as surface-enhanced Raman scattering (SERS). We find that the microstructures exhibit an SERS enhancement factor of >105 and demonstrate remote writing of well-defined Au microstructures within a microfluidic channel for the SERS detection of gaseous molecules. We showcase in situ SERS monitoring of gaseous 4-methylbenzenethiol and real-time detection of multiple small gaseous species with no specific affinity to Au. This one-step, laser-induced fabrication of AuNP microstructures ignites a plethora of possibilities to position desired patterns directly onto or within most surfaces for the future creation of multifunctional lab-on-a-chip devices. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Nanyang Technological University Accepted version X.Y.L. is thankful for financial support from the Singapore Ministry of Education, Tier 1 (Grant RG21/16) and Tier 2(Grant MOE2016-T2-1-043). M.R.L. and C.S.L.K. are thankful for support from the Nanyang President’s Graduate Scholarship. H.K.L. and C.L.L. are thankful for support from an A*STAR graduate scholarship. The authors also thank Mr. Lim Poh Chong from IMRE, A*STAR, for his help with the XRD measurements.

We demonstrate a molecular-level observation of driving CO2 molecules into a quasi-condensed phase on the solid surface of metal nanoparticles (NP) under ambient conditions of 1 bar and 298 K. This is achieved via a CO2 accumulation in the interface between a metal–organic framework (MOF) and a metal NP surface formed by coating NPs with a MOF. Using real-time surface-enhanced Raman scattering spectroscopy, a >18-fold enhancement of surface coverage of CO2 is observed at the interface. The high surface concentration leads CO2 molecules to be in close proximity with the probe molecules on the metal surface (4-methylbenzenethiol), and transforms CO2 molecules into a bent conformation without the formation of chemical bonds. Such linear-to-bent transition of CO2 is unprecedented at ambient conditions in the absence of chemical bond formation, and is commonly observed only in pressurized systems (>105 bar). The molecular-level observation of a quasi-condensed phase induced by MOF coating could impact the future design of hybrid materials in diverse applications, including catalytic CO2 conversion and ambient solid–gas operation. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) Accepted version X.Y.L. thanks for financial support the National Research Foundation, Singapore (NRF-NRFF2012-04), Nanyang Technological University’s start-up grant, and Singapore Ministry of Education, Tier 1 (2016-T1-001-101) and Tier 2 (MOE2016-T2-1-043) grants. H.K.L. appreciates the A*STAR Graduate Scholarship from A*STAR, Singapore. C.S.L.K. acknowledges support from Nanyang Presidential Graduate Scholarship from Nanyang Technological University. J.V.M. and C.K.T. acknowledge support from Boston College and the NSF (CHE 1566445).

Controlling sub-10 nm ligament sizes and open-shell structure in nanoporous gold (NPG) to achieve strained lattice is critical in enhancing catalytic activity, but it remains a challenge due to poor control of reaction kinetics in conventional dealloying approach. Herein, a ligament size-controlled synthesis of open-shell NPG bowls (NPGB) through hetero-epitaxial growth of NPGB on AgCl is reported. The ligament size in NPGB is controlled from 6 to 46 nm by varying the hydroquinone to HAuCl4 ratio. The Williamson-Hall analysis demonstrates a higher lattice strain in smaller ligament size. In particular, NPGB with 6 nm (NPGB 6) ligament size possess the highest strain of 15.4 × 10(-3) , which is nearly twice of conventional 2D NPG sheets (≈8.8 × 10(-3) ). The presence of high surface energy facets in NPGBs is also envisaged. The best electrocatalytic activity toward methanol oxidation is observed in NPGB 6 (27.8 μA μg(-1) ), which is ≈9-fold and 3-fold higher than 8 nm solid Au nanoparticles, and conventional NPG sheets. The excellent catalytic activity in NPGB 6 is attributed to the open-shell structure, lattice strain, and higher electro-active surface area, allowing efficient exposure of catalytic active sites to facilitate the methanol oxidation. The results offer a potential strategy for designing next generation electrocatalysts.