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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.

Plasmonic nanostructures have been widely used in photochemical reactions to enhance reaction rates, decrease energy barriers, or change reaction pathways, but the detailed mechanisms of these plas...

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

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.

Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) with three excitation wavelengths in combination with in situ electrochemical scanning tunneling microscopy (EC-STM) and absorption measure...

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 ( E0') 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 E0' map. In addition, statistical analysis of the E0' map identified a statistically significant 4 mV difference in E0' on Au vs ITO. Electrochemical heterogeneity was also evident in the E0' distribution, as a bimodal distribution was observed in E0' on polycrystalline ITO, but not on gold. A direct comparison between an AFM friction image and the E0' map resolved the electrochemical behavior of individual ITO grains with a spatial resolution of ∼40 nm. The variation in E0' 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.

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 (ΔT = 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.

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 (Tm ) 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 Tm window over which these structures are light-responsive.

Atomic layer deposition (ALD) of TiO2 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 TiO2 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 calc...

Tip functionalization in scanning tunneling microscopy (STM) has become a routine strategy to improve the imaging resolution. Under ultrahigh vacuum and low-temperature conditions, individual molecule(s) can be transferred between the substrate and the tip when proper bias potential is applied, allowing for molecular-scale manipulation and controlled single-bond formation. However, little work related to tip functionalization in electrochemical (EC) environments has been carried out. To bridge that gap, this paper generalizes tip functionalization strategies from UHV conditions to the solid–liquid interfaces. Specifically, we demonstrate the bias-induced functionalization of an Ag tip with cobalt phthalocyanine molecules in EC-STM. Tip-enhanced Raman spectroscopy is employed for the operando detection of the molecule(s) on the tip. We also show that the tip functionalization process may be reversed in some cases, regaining a pristine tip. The control over molecular transfer between the substrate and the t...

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.

We propose the use of Fabry–Perot cavities as a means to augment the enhancement factor in surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) experiments by optical interference. Using both clean-room and bench-top fabrication approaches, we demonstrate that such a design can be readily realized and provides an additional 12× SERS enhancement and 5× TERS enhancement, in good agreement with expectations from electromagnetic modeling. The mechanism of optical interference enhancement is of far-field nature and is independent of the enhancement mechanisms relying on plasmonic and molecular resonances. Therefore, the Fabry–Perot cavity substrate can be applied generally without material and molecular limitations. The Fabry–Perot cavity structure provides enhancement at large incidence angles away from the surface normal, particularly suitable for low-light Raman measurements with side illumination.

The ability to study electron transfer reactions at the solid–liquid interface with nanometer resolution has the potential to critically improve our understanding of electrocatalytic processes. However, few techniques are capable of studying electrode surfaces in situ at the nanoscale. We study the redox reactions of Nile Blue (NB) covalently tethered to an Au(111) electrode using in situ tip-enhanced Raman spectroscopy (TERS) and show that TERS amplitude decreases reversibly as NB is reduced. The potential dependent TERS intensity allows us to associate an electrochemical wave with the loss of electronic resonance of NB and another with the peak of fluorescence of tethered NB, which we tentatively attribute to the disassembly of on-surface NB aggregates. The study of the electrochemical activity of immobile adsorbates at the solid–liquid interface with TERS is an essential step toward the realization of in situ spectroscopic mapping at the nanoscale.

Vibrational coherence in the metal–metal-to-ligand-charge transfer (MMLCT) excited state of cyclometalated platinum dimers with a pseudo C2 symmetry was investigated where two nearly degenerate transitions from the highest occupied molecular orbital (metal–metal σ* orbital) to higher energy ligand π* orbitals could be simultaneously induced. We observed oscillatory features in femtosecond degenerate transient absorption (TA) signals from complexes [(ppy)Pt(μ-tBu2pz)]2 (1) and anti-[(ppy)Pt(μ-pyt)]2 (2), which are attributed to coherent nuclear motions that modulate the HOMO (antibonding σ*) energy level, and hence, the energy for the MMLCT transition. The characteristics of such coherent nuclear motions, such as the oscillatory frequency and the dephasing time, differ between 1 and 2 and are explained by mainly two structural factors that could influence the vibrational coherence: the Pt–Pt distance (2.97 A for 1 vs 2.85 A for 2) and molecular shape (1 in an “A” frame vs 2 in an “H” frame). Because the el...

Ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) is used to investigate adsorption of molecular oxygen (O2) 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 O2/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 18O–18O stretching frequency at 1151 cm–1 for O2/CoPc/Ag(111), and Co–16O and Co–18O 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 probin...

Single crystal SrTiO3 nanocuboids having primarily TiO2-(001) surfaces and nanododecahedra having primarily (110) surfaces were created by two separate hydrothermal synthesis processes. Pd nanoparticles grown on the two sets of STO nanopolyhedra by atomic layer deposition show different morphologies and CO oxidation performance. Transmission electron microscopy and small-angle X-ray scattering show that 2–3 nm Pd nanoparticles with 3–5 nm interparticle distances decorate the STO surfaces. When the number of ALD cycles increases, the growth of the Pd nanoparticles is more significant in size on TiO2-(001)-STO surfaces, while that on (110)-STO surfaces is more predominant in number. High resolution electron microscopy images show that single crystal and multiply twinned Pd nanoparticles coexist on both types of the STO nanopolyhedra and exhibit different degrees of adhesion. The CO oxidation reaction, which was employed to determine the dependence of catalytic activity, showed that the Pd catalytic performa...

Surface-enhanced Raman spectroscopy probes adsorbates on a plasmonic substrate and offers high sensitivity with molecular identification capabilities. In this study, we present a refined methodology for considering the supporting substrate in the computation of the Raman spectra. The supporting substrate is taken into account by employing a periodic slab model when doing the geometry optimization and normal mode analysis, and then the Raman spectrum is calculated for the isolated molecule but with the normal modes from the surface structure. We find that the interaction with the surface induces internal distortion in the molecule, and spectral shifts in the computed Raman spectrum. By comparing a low temperature surface-enhanced Raman spectroscopy measurement of Rhodamine 6G (R6G) with the computed Raman spectra of a series of adsorption geometries, we propose that the binding state captured in the experiment tends to possess the least internal distortion. This binding state involves upward orientation of...

Surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS) is a technique that has the potential to study the ultrafast dynamics of surface adsorbates with simultaneously high temporal and spectral resolutions. One of the greatest impediments to the further development of SE-FSRS is the lack of high-quality plasmonic substrates that permit excitation and prove wavelength agility as well as flexible selection of the adsorbate molecule. Herein, we present a simple aggregation/stabilization procedure for the formation of gold nanosphere oligomers with tunable plasmonic properties and flexible choice of adsorbate that are ideal for SE-FSRS and other coherent Raman spectroscopies.

Lithographic and nonlithographic two-dimensional (2D) substrates for surface-enhanced Raman spectroscopy (SERS) have gained enormous popularity as analytical platforms for detection and identification of various analytes. However, their near- and far-field properties in most cases remain poorly understood. We have previously developed a metal nanopillar film over nanospheres (FON) platform exhibiting Raman enhancement factors of ∼107. These substrates have a reproducible and predictable localized surface plasmon resonance throughout the entire visible region and much of the near-IR region of the electromagnetic spectrum. Extending upon these results, we have utilized wavelength-scan surface-enhanced Raman excitation spectroscopy to unravel the relationship between near- and far-field properties of FON surface-enhanced Raman spectroscopy substrates. We examined by scanning electron microscopy FONs fabricated by either stationary (ST-FONs) or spun (SP-FONs) metal deposition to examine the interrelationships...

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.

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.

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., Funding is acknowledged from the European Research Council (ERC Advanced Grant No. 787510-4DBIOSERS to L.M.L.-M., ERC Advanced Grant No. 789104-eNANO to F.J.G.A., ERC Starting Grant No. 259432-MULTIBIOPHOT to J.K., ERC Consolidator Grant No. 772108-DarkSERS); the Department of Education of the Basque Government (Grant No. IT1164-19 to J.A.); the Spanish MINECO (CTQ2017-88648-R to R.A.-P., MAT2016-77809-R to I.P.-S. and J.P.-J.); the EPSRC (EP/P034063/1 to S.B., EP/L027151/1 to J.B., EP/L014165/1 to D.G. and K.F.); IDUN-Danish National Research Foundation (DNRF122) and Villum Fonden (Grant No. 9301) to A.B.; the National Research Foundation of Korea (Grant No. 2019R1A2C3004375 to J.B.); the German Science Foundation, DFG (SFB 1278 Polytarget (Project B4) to V.D., Grant No. SCHL 594/13-1 to S.S., Germany’s Excellence Strategy (EXC 2089/1-390776260) to S.M.); the Federal Ministry of Education and Research, Germany (BMBF) (Grant InfectoGnostics 13GW0096F to D.C.-M. and J.P.); DARPA-16-35-INTERCEPT-FP-018 to L.F.; the UK BBSRC (Grant No. BB/L014823/1 to R.G.); the Department of Science and Technology (DST Nanomission Project SR/NM/NS-23/2016 to K.G.T.); the U.S. National Science Foundation (Grant No. CHE-1707859 to A.J.H., Center for Sustainable Nanotechnology CHE-1503408 (Centers for Chemical Innovation Program) to C.L.H., Center for Chemical Innovation Chemistry at the Space-Time Limit (CaSTL) CHE-1414466 to G.C.S. and R.P.V.D., Grant No. CHE-1807269 to K.A.W.); the Knut and Alice Wallenberg Foundation to M.K.; the Office of Naval Research (Grant No. N00014-18-1-2876 to N.A.K.); Royal Society of New Zealand Te Apa̅rangi to E.L.R. and B.A.; Singapore Ministry of Education, Tier 1 (RG11/18) to X.Y.L.; the Photoexcitonix Project in Hokkaido Univ., Japan, to K.M.; BioNano Health-Guard Research Center funded by the Ministry of Science and ICT (MSIT) of Korea as Global Frontier Project (Grant No. H-GUARD_2013M3A6B2078947) to J.-M.N.; NSFC of P. R. China (Grant No. 21705015 to Y.O., Grant No. 21633005 to B.R.); National Key R&D Program (2017YFA0206902) to C.X. This work was coordinated under the Maria de Maeztu Units of Excellence Program from the Spanish State Research Agency—Grant No. MDM-2017-0720.

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 (S1) 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 O2/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 O2 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.

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.

The field of ultrafast surface-enhanced Raman spectroscopy (SERS) is rapidly expanding; however, few applications for these new techniques have been demonstrated. One obstacle for the widespread application of ultrafast SERS is the addition of highly enhancing and scattering plasmonic substrates to already complex nonlinear spectroscopies. The competition between extinction and enhancement in ultrafast SERS techniques complicates the optimization of a number of experimental parameters. Here we study the concentration and path length dependences of signal quality in surface-enhanced femtosecond stimulated Raman spectroscopy (SE-FSRS). We find that in contrast to previous studies of spontaneous SERS which use signal magnitudes to define optimal experimental parameters, signal-to-noise ratios (SNRs) are the best measure of ideal experimental parameters in SE-FSRS. We report ideal concentrations and path lengths to use in transmissive geometry SE-FSRS experiments with colloidal nanoparticle substrates. Our re...

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, E0′, 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 E0′ at each spatial location. Histograms of single-molecule E0′ 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 qua...

Probing the electrochemistry of single molecules is a direct pathway toward a microscopic understanding of a variety of electron transfer processes related to energy science, such as electrocatalysis and solar fuel cells. In this context, Zaleski et al. recently studied the single electron transfer reaction of the dye molecule rhodamine-6G (R6G) by electrochemical single molecule surface-enhanced Raman spectroscopy (EC-SMSERS) (J. Phys. Chem. C 2015, 119, 28226−28234). In that work, the reductions of the dye molecule R6G were not only observed in the same potential range as in the ensemble surface cyclic voltammogram but also seen under some less negative potentials. Aiming to understand and explain this experiment theoretically, we relate the binding energy of R6G+ adsorbed on a silver nanoparticle (AgNP) to its reduction potential and further use periodic density functional theory to calculate this adsorption energy at different local surface sites. Well-defined crystal facets and defective surfaces, ar...

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.

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 (H2TBPP) on a Cu(111) surface. At room temperature, the barrier between the porphyrin ring buckled up/down conformations of the H2TBPP-Cu(111) system is easily overcome, and a 2.6 A 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 Angstrom-scale spatial resolu...

The combination of electrochemistry (EC) and single molecule surface-enhanced Raman spectroscopy (SMSERS) has recently proven to be a sensitive method to investigate electron transfer (ET) reactions at the single molecule level. SMSERS can both detect single redox-active molecules and potentially monitor both the oxidized (O) and reduced (R) forms of a one-electron ET reaction in a single experiment. Herein, we report progress toward complete monitoring of single ET reactions with EC-SMSERS. We first obtained the solution phase resonance Raman (RR) spectrum of the Rhodamine 6G (R6G) neutral radical (R) with thin-layer resonance Raman spectroelectrochemistry (EC-RRS). The experimental spectrum was then correlated with the spectrum calculated by density functional theory (DFT). We then describe our approach to address the problem of adsorbate (R) loss caused either by desorption or reaction of the neutral radical with trace water or oxygen during the EC-SMSERS experiment. We have investigated a covalent cro...

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.

Tip-enhanced Raman spectroscopy (TERS) is a powerful technique that integrates the vibrational fingerprinting of Raman spectroscopy and the sub-nanometer resolution of scanning probe microscopy (SPM). As a result, TERS is capable of obtaining chemical maps of analyzed specimens with exceptional lateral resolution. This is extremely valuable for the study of interactions between molecules and substrates, in addition to structural characterization of biological objects, such as viruses and amyloid fibrils, 2D polymeric materials, and monitoring electrochemical and photo-catalytic processes. In this mini-review, we discuss the most significant advances of TERS, including: super high resolution chemical imaging, monitoring of catalytic processes, incorporation of pulsed-excitation techniques, single-site electrochemistry, biosensing, and art conservation. We begin with a short overview of TERS, comparing it with other surface analytical techniques, followed by an overview of recent developments and future applications in TERS.

Experimental results of a smooth composition gradient within Pt/Pd alloy nanoparticles with Pt enrichment at the corners are reported. We find that the Pt concentration gradually increases toward the outermost surface, and it appears different from the thermodynamically most stable configuration. We demonstrate that it is the result of reaction kinetics of both the reduction of precursors and growth of nanoparticles, by a growth model. We then explain that the corner Pt enrichment is a result of local thermodynamic control at the corners. This mixed control of kinetics and thermodynamics in bimetallic nanoparticle synthesis can lead to the formation of particles with a complex concentration profile, which could be interesting when these particles are used in catalytic applications. Our analysis is not simply a qualitative model but rather a relatively rigorous quantitative analysis of the composition as a function of the growth conditions, which can serve as a basis for improved reproducibility of synthes...

Recent methodological and instrumental advances have afforded surface-enhanced Raman spectroscopy (SERS) wide recognition in the cultural heritage field as a powerful tool for the analysis of organic colorants. The many advantages of SERS are counterbalanced by the fact that it has limited ability to resolve dye mixtures. In this work, a systematic study was conducted to determine the technique’s ability to discriminate closely related molecules in binary mixtures for a selection of natural red dyes widely used in painting and textile dyeing (alizarin, purpurin, carminic and laccaic acids, brazilein). Spectra were recorded on two different metal substrates, i.e., citrate-reduced silver colloids and silver films over nanospheres (AgFONs). After working on reference systems, the study was expanded to include analysis of red lake oil paint reconstructions upon hydrolysis with hydrofluoric acid, aiming to gain a deeper understanding of how varying the experimental conditions may affect dye identification. It ...

Over the past few years, ricin has been discussed frequently because of letters sent to high-ranking government officials containing the easily extracted protein native to castor beans. Ricin B chain, commercially available and not dangerous when separated from the A chain, enables development of ricin sensors while minimizing the hazards of working with a bioterror agent that does not have a known antidote. Recent events have increased the risk of ricin exposure for civilians, and there is a need for rapid, real-time detection of ricin. To this end, aptamers have been used recently as an affinity agent to enable the detection of ricin in food products via surface-enhanced Raman spectroscopy (SERS) on colloidal substrates. One goal of this work is to extend ricin sensing into human whole blood; this goal requires application of a commonly used plasmonic surface, the silver film-over-nanosphere (AgFON) substrate, which offers stable SERS enhancement factors of 106 in human whole blood. Herein, this aptamer...

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.

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.

We employ SrTiO3 nanocuboid single crystals with well-defined (001) surfaces that are synthesized to have either a TiO2- or SrO-terminated surface to investigate the influence of surface termination on the morphology and the chemical property of supported metallic nanoparticles. Using such monodispersed STO nanocuboids allows for practical catalytic reaction studies as well as surface studies comparable to a single crystal model catalyst. Pd nanoparticles were grown by atomic layer deposition, which is able to control the effective coverage, chemical state, and the size of the Pd nanoparticles. The properties of Pd nanoparticles were examined by transmission electron microscopy, X-ray absorption spectroscopy, and X-ray photoemission spectroscopy. The morphology and growth pattern for the Pd nanoparticles supported on the SrTiO3 nanocuboids are shown to depend on the surface termination.

Verification of single-molecule (SM) detection for surface-enhanced Raman spectroscopy (SERS) requires the use of two analytes via either the bianalyte or isotopologue approach. For both approaches, the preferential observation of the individual analytes over a combination of both analytes is used to conclude that SM detection has been achieved. Isotopologues are preferred because they have identical surface binding affinities and Raman cross sections, whereas bianalyte pairs typically do not. We conducted multianalyte SERS studies to investigate the limitations of the bianalyte approach. The bianalyte partners, Rhodamine 6G (R6G-d0) and crystal violet (CV-d0), were directly compared, while SM detection was verified (or disproved) using their corresponding isotopologues (R6G-d4, CV-d12). We found that the significant difference in counts between R6G and CV can provide misleading evidence for SMSERS. We then rationalized these results using a joint Poisson-binomial model with unequal detection probabilitie...

This work demonstrates for the first time the capability of measuring surface vibrational spectra for adsorbates during atomic layer deposition (ALD) reactions using operando surface-enhanced Raman spectroscopy (SERS). We use SERS to study alumina ALD growth at 55 °C on bare silver film-over nanosphere (AgFON) substrates as well as AgFONs functionalized with thiol self-assembled monolayers (SAMs). On bare AgFONs, we observe the growth of Al–C stretches, symmetric C–H and asymmetric C–H stretches during the trimethylaluminum (TMA) dose half-cycle, and their subsequent decay after dosing with H2O. Al–C and C–H vibrational modes decay in intensity with time even without H2O exposure providing evidence that residual H2O in the ALD chamber reacts with −CH3 groups on AgFONs. The observed Al–C stretches are attributed to TMA dimeric species on the AgFON surface in agreement with density functional theory (DFT) studies. We observe Al–C stretches and no thiol vibrational frequency shifts after dosing TMA on AgFONs...

Atomic layer deposition (ALD) of TiO

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.

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.