Preface to the special issue dedicated to Professor Richard P. Van Duyne (1945–2019)

Abstract

It is our great honor and pleasure to dedicate this special issue of the Journal of Raman Spectroscopy to Professor Richard P. Van Duyne (1945–2019). Van Duyne is well known for his discovery and advancement of surface-enhanced Raman scattering (SERS), thereby creating a field of research that continues to be vibrant over 40 years after its initial discovery. This issue includes papers that cover the three main research directions related to SERS, namely, theory, methodology, and applications, and it also features the fruitful abundance of SERS applications that have been developed in bioanalysis, biosensing, pharmaceutical analysis, food safety, environmental safety, electrochemistry, materials, and catalysis. The following figure shows the growth of the SERS field, using a number of publications as the metric, since 1997. In the past 5 years, over 2000 SERS-related manuscripts have been published each year. The 32 invited articles in this issue are from Richard's former students, collaborators, and friends. These research results paint an exciting picture of modern SERS as a fast-developing methodology in both fundamental and applied scientific research. A highlight of this issue is the first paper, written by Prof. George C. Schatz. As a long-term friend and the most intimate collaborator of Van Duyne, Schatz reviews and summarizes Van Duyne's significant contributions to SERS and other related fields. Starting with a short biography, Schatz then reviews Van Duyne's leading contributions to the discovery and development of SERS as well as pioneering work on localized surface plasmon resonance (LSPR), tip-enhanced Raman scattering (TERS), electrochemical surface-enhanced Raman scattering (SERS)/TERS, and ultrafast nonlinear techniques.[1] The second paper by Moskovits and Piorek is also important, giving a brief history of SERS and its connection with localized surface plasmon excitation. Being a pioneer in understanding the surface plasmon's role in SERS, Moskovits' retrospective about the discovery of SERS and the development of the electromagnetic enhancement mechanism is highly valuable for all readers, especially for newcomers to this field.[2] The other 30 articles are classified into the three SERS research themes mentioned earlier: theory, methods, and applications. However, some articles cover more than one theme, as is appropriate for this interdisciplinary topic. Indeed, synergistic efforts have played a key role in advancing SERS research. Khlebtsov and Le Ru derive analytic expressions for the surface- and orientation-averaged SERS enhancement factors (EFs) of randomly oriented small plasmonic spheroidal particles. The expressions improve electrostatic approximations for spheroids and are in full agreement with T-matrix calculations for far-field cross sections and near-field SERS EFs over a much wider range of nanoparticle sizes. The authors also discuss possible ways to generalize their analytic solutions and apply them to other systems with axially symmetrical particles.[3] Aizpurua et al. introduce a general treatment to simulate single-molecule TERS spectra and the corresponding energy-filtered vibrational fingerprint maps, wherein the polarization properties of the single molecule and that of the optical enhancing nanoresonator can be calculated separately and then conveniently combined to obtain the total Raman cross section of the molecule when there is a strongly inhomogeneous field.[4] Reich et al. review a number of experimental studies examining the predictions of the electromagnetic enhancement theory of SERS and focusing on the results that require going beyond simple electromagnetic enhancement theory. The authors argue that the results of these experiments reveal the need to develop the theory of SERS further, especially the exact role of plasmonic enhancement in inelastic light scattering.[5] Ziegler et al. elaborate on the original Raman signals of protoporphyrin IX (PPIX) and the chemical enhancement contributions to the observed SERS spectra with time dependent density functional theory (TDDFT) calculations for Au2/Ag2-PPIX complexes. A simple method for determining the importance of the transition moment normal mode dependence relative to Frank–Condon factors (A-term) is implemented, and the results of the vibronic analysis observed on the overtones/combination bands in the SERS spectrum are discussed.[6] Schatz et al. introduce a method for fabricating localized electrochemical SERS probes based on nanopipettes and electrodeposition. In situ control of the surface potential brings advantages that help to mitigate surface oxidation and substrate-analyte incompatibilities. Versatile electrochemical SERS platforms with tailored nanostructure and chemical features can be conveniently constructed on nanopipettes by adapting the method to meet different experimental requirements.[7] Chikkaraddy, Baumberg, et al. study the dynamics of single bonds through SERS from single SERS-marker molecules containing a distinctive single alkyl bond. Assembly of the nanogaps and positioning of single molecules inside the EM hot spot are precisely controlled using DNA origami constructs. The results confirm the role of picocavities in this nanogap geometry, allowing observation of SERS signatures from individual vibrating bonds.[8] Litti, Liz-Marzán, et al. propose a protocol, with an associated smartphone application (known as SERSTEM), which enables users to determine the average SERS intensity per nanoparticle from transmission electron microscopy (TEM) and SERS data. As a proof of concept, they demonstrated the method for Au nanostars and nanorods, carrying four different Raman reporters, and implemented in the SERSTEM App, which is publicly available from an open-source platform.[9] Zhang, Dong, et al. study the formation of ordered supramolecular self-assembly substrates based on cytosine and 4,4-bipyridine adsorption on Ag(111) with subnanometer-resolved TERS using a low-temperature ultrahigh-vacuum scanning tunneling microscope (STM). The combination of sub-nanometer resolution TERS and STM imaging reveals interactions between the two-component samples and nearby molecules. They show that the TERS signal is sensitive to local molecular interactions and can provide new opportunities to monitor molecular processes and probe nanoscale surface chemistry and photocatalysis.[10] Oh, Nam, et al. in their minireview, introduce key issues for quantitative SERS and present the fundamental SERS features obtained by single-particle analysis. They categorize the nanogap particle-based SERS platforms into two different classes: plasmonic nanogap structures with an intergap and plasmonic nanogap structures with an intragap. They discuss the challenges and perspectives in designing and synthesizing nanogap structures that deliver strong, reproducible, and reliable SERS signals for quantitative SERS analysis.[11] In the work presented by Xu, Bell, et al., a SERS-active superhydrophobic (SHP) tip is constructed by an adsorbing a layer of densely packed and uniform plasmonic nanoparticles onto a hydrophilic tip. The resulting particle-tipped needles allow dual enhancement of the Raman signals from microdroplets of low concentration analytes by combining analyte enrichment through solvent evaporation and plasmonic SERS enhancement. The combination of small sample volume, preconcentration, and SERS allows extremely low total amounts of analytes to be detected.[12] Kneipp et al. introduce the study of different molecules using surface-enhanced hyper Raman spectroscopy (SEHRS) with citrate-stabilized Ag and Au nanoparticles. SEHRS spectra of four reported molecules: 2-NAT, pATP, pNTP, and crystal violet, as well as desipramine, and imipramine, were obtained at an excitation wavelength of 1550 nm. The two-photon excited spectra of three types of SEHRS labels containing 2-NAT as a probe molecule inside macrophage cells show that molecules in the cellular environment can also be observed. The possibility of using shortwave infrared excitation with gold nanostructures has important implications for the utilization of SEHRS in bio-probing and sensing.[13] Frontiera et al. adapt their previously developed stimulated Raman-based imaging technique for a 2.04-MHz laser system to improve the biological capabilities of the setup. The adaptation demonstrates strong signal depletion and resolution enhancements, which is comparable with the metrics obtained with the 1-kHz laser. Further improvements in resolution are however not achieved. They expect that the observed inconsistent depletion is a result of the inconsistent pulse profile and conclude that efficient depletion depends on highly reproducible and stable laser pulses.[14] Baumberg, Nijs, et al. show that highly reproducible SERS spectra can be achieved with relative standard deviations below 1% by carefully controlling the parameters connected to the signal generation, from particle gap sizes to aggregation times. In addition, they show that sources of variance in substrates can be turned into tuning parameters, allowing for control in the sensitivities of the substrates when carefully characterized and controlled.[15] Ikeda et al. introduce a strategy for utilizing long-range surface plasmons (LRSPs) on a highly dampening platinum surface to study its electrochemical SERS properties. Electrochemical SERS was demonstrated using a Pt film electrode with a thickness of 30 nm and 632.8 nm excitation wavelength. Also, by utilizing the LRSP mode, electrochemical reactions on the Pt surface could be protected from any nanoscale effects, which is advantageous over conventional SERS.[16] Murakoshi et al. carry out electrochemical SERS measurements to clarify the detailed molecular behavior in the strong coupling regime. The results provide information about the change in the distance between the molecules and the metal surface at the angstrom level, revealing the origin of the potential dependence on the coupling strength.[17] Li, Tian, et al. prepare graphene-coated Au ([email protected]) nanoparticles via chemical vapor deposition (CVD). The graphene shell thickness could be controlled from a few layers to multilayers, and the [email protected] SERS activities are characterized using mercaptobenzoic acid as a probe molecule. Both the pH and high-temperature stabilities of the [email protected] nanoparticles are characterized. Also, [email protected] satellite structures are developed via a self-assembly method.[18] Ding, Tian, et al. propose a hierarchical structure for ATR c-SHINERS to further improve the sensitivity of SHINERS by up to 2 orders of magnitude by employing an ATR prism coupled with the SHINs-substrate system. The additional enhancement for flat nonmetallic or metallic substrates is essential important for a wide array of practical applications.[19] Cialla-May, Popp, et al. show an easy, rapid, and nondestructive method for SERS detection of a B12-containing enzyme of the respiratory chain in bacterial membranes. The specific detection of the tetrachloroethene reductive dehalogenase is helpful for monitoring the active dehalogenation processes of bacteria in a low oxygen environment.[20] Ji, Ozaki, et al. review the recent advances in SERS-based sensors for the determination of inorganic ions, with emphasis on the sensing mechanism and methods. In addition, the review briefly introduces current challenges and future research opportunities for SERS in inorganic ion sensing.[21] Haynes et al. provide a systematic investigation of how the three major metal film over nanosphere (FON) substrate fabrication parameters—nanosphere size, deposited metal thickness, and metal choice—impact the resulting LSPR. They employ optimized FONs for the sensing of an important allergen, soybean agglutinin with a limit of detection of 10 μg/ml.[22] Zhao et al. investigate the Raman enhancement mechanism of the 1,2-bis (4-pyridyl) ethylene (BPE) on the PbI2. The results prove that the BPE molecule enters the lattice of PbI2 and that the PbI2 is a special adsorbent, demonstrating and providing a better understanding of the Raman enhancement mechanism of PbI2.[23] Haes et al. evaluate the influence of the nanostar morphology and plasmonic properties on the SERS detection of uranyl. LSPR spectra with up to three resonances are observed, and all three optical features are found to arise from synergistic coupling between the nanostar core and branches. Similar effects may be exploited in the future for detection of trace molecules using Au nanostars.[24] Choo et al. develop a novel SERS-active MOF-gold nanoparticle (AuNP) substrate for SERS and provide a rapid and sensitive method for the determination of the total iron binding capacity of transferrin (Tf) in human serum for diagnosing iron deficiency. By monitoring variations in the intensity of Raman signals for the MOF-AuNP complexes with the Tf concentration, Tf is quantitatively estimated and the limit of detection is determined to be 0.51 μM in spiked human serum.[25] Brolo et al. utilize Raman maps and histograms to analyze carbon materials and films and to describe heterogeneity in the electrochemically obtained carbon films. The method used herein could be used to evaluate the effects of manufacturing parameters and to infer the quality of the synthesized carbon material or film.[26] Zhang, Tong, et al. study the helicity-resolved Raman scattering of layered WS2 excited by circularly polarized light. The results indicate that excitons not only affect the intensity of Raman peaks but also change the Raman selection rules, which play an important role in resonant Raman spectra. This work provides a better understanding of the role of different electron–phonon and exciton–phonon coupling in the Raman scattering spectrum and indicates that helicity-resolved resonant Raman spectroscopy (HRRS) can be used to study the role of excitons in Raman scattering.[27] Duan et al. introduce a strategy for Ag-Au nanowire (NW) fabrication by virtue of interfacial self-assembly of Ag NWs in plasmonic nanoparticles and galvanic replacement reactions. The two-dimensional Ag–Au NW arrays are used as SERS substrates for the detection of thiram and melamine with limits of detection of 1 and 10 nM, respectively.[28] Vo-Dinh et al, in their review, provide an overview of the development and application of SERS and plasmonic nanoplatforms in the Vo-Dinh laboratory in recent years, including inverse molecular sentinels utilizing gold nanostars and nanorattle DNA sensors. These platforms have been used for a wide variety of applications, such as biosensing, diagnostics, and therapy, that contribute to expanding the field of SERS and underline the impact and potential of plasmonic systems.[29] Pazos-Perez, Alvarez-Puebla, et al. review the different strategies used to produce homogeneous nanoparticle assemblies supported on colloidal templates for different applications.[30] Tay et al. fabricate paper-based SERS sensors through drop-casting and inkjet printing colloidal Ag or Au nanoparticles onto cellulose-based filter papers. The paper SERS sensors exhibit excellent batch-to-batch uniformity, and the biggest advantage they provide is point-of-sampling capability. They also demonstrate the use of paper SERS sensors for the detection of chemical aerosols.[31] Jiang et al. demonstrate an alternative way to reuse and recycle a plasmonic tip for distinct molecular systems inside an ultrahigh vacuum. They provide evidence of the ability to recycle tips without compromising the TERS experimental results and a long-term preservation (>2 months) of plasmonically active probes inside ultrahigh vacuum is demonstrated.[32] Finally, it is our great pleasure to be the guest editors of this special issue. We are very grateful to all the authors for their contributions and splendid cooperation during editing and especially to Laurence A. Nafie, the Editor-in-Chief, for his kind help and expert suggestions throughout this process. Also, we are deeply indebted to all the reviewers for their prompt and devoted professional evaluation, critical to maintaining quality of the issue. Finally, we express our gratitude to Prof. Richard P. Van Duyne, for inspiring all of the great work presented here.