Minimum Enhancement of Surface-Enhanced Raman Scattering for Single-Molecule Detections

Photovoltaic cells as a highly efficient system for biomedical and electrochemical surface-enhanced Raman spectroscopy analysis

Surface-enhanced Raman scattering (SERS) has been intensively used recently as a highly sensitive, non-destructive, chemical specific, and label-free technique for a variety of studies. Here, we present a novel SERS substrate for: (i) the standard ultra-trace analysis, (ii) detection of whole microorganisms, and (iii) spectroelectrochemical measurements. The integration of electrochemistry and SERS spectroscopy is a powerful approach for in situ investigation of the structural changes of adsorbed molecules, their redox properties, and for studying the intermediates of the reactions. We have developed a conductive SERS platform based on photovoltaic materials (PV) covered with a thin layer of silver, especially useful in electrochemical SERS analysis. These substrates named Ag/PV presented in this study combine crucial spectroscopic features such as high sensitivity, reproducibility, specificity, and chemical/physical stability. The designed substrates permit the label-free identification and differentiation of cancer cells (renal carcinoma) and pathogens (Escherichia coli and Bacillus subtilis). In addition, the developed SERS platform was adopted as the working electrode in an electrochemical SERS approach for p-aminothiophenol (p-ATP) studies. The capability to monitor in real-time the electrochemical changes spectro-electro-chemically has great potential for broadening the application of SERS.

Enhancements inside and outside the junctions of Ag colloidal dimers

SERS signal enhancements inside and outside the junctions of the dimers were experimentally calculated.

Highly reproducible surface-enhanced Raman scattering substrate for detection of phenolic pollutants

The ordering degree of nanostructures is the key to determining the uniformity of surface-enhanced Raman scattering (SERS). However, fabrication of large-area ordered nanostructures remains a challenge, especially with the ultrahigh-density (>10¹⁰ cm^-2). Here, we report a fabrication of large-area ultrahigh-density ordered Ag@Al₂O₃/Ag core-shell nanosphere (NS) arrays with tunable nanostructures. The ultrahigh-density (2.8 × 10¹⁰ cm^-2) ordered NS arrays over a large-area capability (diameter >4.0 cm) enable the uniform SERS signals with the relative standard deviation of less than 5%. The as-fabricated highly reproducible SERS substrate can be applied to detect trace phenolic pollutants in water. This work does not only provide a new route for synthesizing the ultrahigh-density ordered nanostructures, but also create a new class of SERS substrates with high sensitivity and excellent reproducibility.

The mechanism of an enzymatic reaction-induced SERS transformation for the study of enzyme–molecule interfacial interactions

The vibrational frequencies and spectral intensity of enzyme-conjugated SERS-active reporter molecules (4-MBA) shift and change regularly as a function of the concentration of glucose.

Preparation of a Superhydrophobic and Peroxidase-like Activity Array Chip for H2O2 Sensing by Surface-Enhanced Raman Scattering

In this paper, we propose a novel and simple method for preparing a dual-biomimetic functional array possessing both superhydrophobic and peroxidase-like activity that can be used for hydrogen peroxide (H2O2) sensing. The proposed method is an integration innovation that combines the above two properties and surface-enhanced Raman scattering (SERS). We integrated a series of well-ordered arrays of Au points (d = 1 mm) onto a superhydrophobic copper (Cu)/silver (Ag) surface by replicating an arrayed molybdenum template. Instead of using photoresists and the traditional lithography method, we utilized a chemical etching method (a substitution reaction between Cu and HAuCl4) with a Cu/Ag superhydrophobic surface as the barrier layer, which has the benefit of water repellency. The as-prepared Au points were observed to possess peroxidase-like activity, allowing for catalytic oxidation of the chromogenic molecule o-phenylenediamine dihydrochloride (OPD). Oxidation was evidenced by a color change in the presence of H2O2, which allows the array chip to act as an H2O2 sensor. In this study, the water repellency of the superhydrophobic surface was used to fabricate the array chip and increase the local reactant concentration during the catalytic reaction. As a result, the catalytic reaction occurred when only 2 μL of an aqueous sample (OPD/H2O2) was placed onto the Au point, and the enzymatic product, 2,3-diaminophenazine, showed a SERS signal distinguishable from that of OPD after mixing with 2 μL of colloidal Au. Using the dual-biomimetic functional array chip, quantitative analysis of H2O2 was performed by observing the change in the SERS spectra, which showed a concentration-dependent behavior for H2O2. This method allows for the detection of H2O2 at concentrations as low as 3 pmol per 2 μL of sample, which is a considerable advantage in H2O2 analysis. The as-prepared substrate was convenient for H2O2 detection because only a small amount of sample was required in each analysis. Highly sensitive detection was realized using SERS. Therefore, this chip was shown to exhibit significant potential for applications in bioanalysis.

Monitoring plasmon-driven surface catalyzed reactions in situ using time-dependent surface-enhanced Raman spectroscopy on single particles of hierarchical peony-like silver microflowers

Investigating the kinetics of catalytic reactions with surface-enhanced Raman scattering (SERS) on a single particle remains a significant challenge. In this study, the single particle of the constructed hierarchical peony-like silver microflowers (SMFs) with highly roughened surface led to the coupling of high catalytic activity with a strong SERS effect, which acts as an excellent bifunctional platform for in situ monitoring of surface catalytic reactions. The kinetics of the reaction of 4-nitrothiophenol (4-NTP) dimerizing into 4,4'-dimercaptoazobenzene (DMAB) was investigated and comparatively studied by using the SERS technique on a single particle of different morphologies of SMFs. The results indicate that a fully developed nanostructure of a hierarchical SMF has both larger SERS enhancement and apparent reaction rate constant k, which may be useful for monitoring and understanding the mechanism of plasmon-driven surface catalyzed reactions.

Single-molecule surface-enhanced Raman spectroscopy

A general overview of the field of single-molecule (SM) surface-enhanced Raman spectroscopy (SERS) as it stands today is provided. After years of debates on the basic aspects of SM-SERS, the technique is emerging as a well-established subfield of spectroscopy and SERS. SM-SERS is allowing the observation of subtle spectroscopic phenomena that were not hitherto accessible. Examples of the latter are natural isotopic substitutions in single molecules, observation of the true homogeneous broadening of Raman peaks, Raman excitation profiles of individual molecules, and SM electrochemistry. With background examples of the contributions produced by our group, properly interleaved with results by other practitioners in the field, we present some of the latest developments and promising new leads in this new field of spectroscopy.

Surface-enhanced Raman spectroscopy (SERS): progress and trends

Surface-enhanced Raman spectroscopy (SERS) combines molecular fingerprint specificity with potential single-molecule sensitivity. Therefore, the SERS technique is an attractive tool for sensing molecules in trace amounts within the field of chemical and biochemical analytics. Since SERS is an ongoing topic, which can be illustrated by the increased annual number of publications within the last few years, this review reflects the progress and trends in SERS research in approximately the last three years. The main reason why the SERS technique has not been established as a routine analytic technique, despite its high specificity and sensitivity, is due to the low reproducibility of the SERS signal. Thus, this review is dominated by the discussion of the various concepts for generating powerful, reproducible, SERS-active surfaces. Furthermore, the limit of sensitivity in SERS is introduced in the context of single-molecule spectroscopy and the calculation of the 'real' enhancement factor. In order to shed more light onto the underlying molecular processes of SERS, the theoretical description of SERS spectra is also a growing research field and will be summarized here. In addition, the recording of SERS spectra is affected by a number of parameters, such as laser power, integration time, and analyte concentration. To benefit from synergies, SERS is combined with other methods, such as scanning probe microscopy and microfluidics, which illustrates the broad applications of this powerful technique.

A scheme for detecting every single target molecule with surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is now a well-established technique for the detection, under appropriate conditions, of single molecules (SM) adsorbed on metallic nanostructures. However, because of the large variations of the SERS enhancement factor on the surface, only molecules located at the positions of highest enhancement, so-called hot-spots, can be detected at the single-molecule level. As a result, in all SM-SERS studies so far only a small fraction, typically less than 1%, of molecules are actually observed. This complicates the analysis of such experiments and means that trace detection via SERS can in principle still be vastly improved. Here we propose a simple scheme, based on selective adsorption of the target analyte at the SERS hot-spots only, that allows in principle detection of every single target molecule in solution. We moreover provide a general experimental methodology, based on the comparison between average and maximum (single molecule) SERS enhancement factors, to verify the efficiency of our approach. The concepts and tools introduced in this work can readily be applied to other SERS systems aiming for detection of every single target molecule.