Clinical SERS: are we there yet?

SERS immuno- and apta-assays in biosensing/bio-detection: Performance comparison, clinical applications, challenges

Surface Enhanced Raman Spectroscopy is increasingly used as a sensitive bioanalytical tool for detection of variety of analytes ranging from viruses and bacteria to cancer biomarkers and toxins, etc. This comprehensive review describes principles of operation and compares the performance of immunoassays and aptamer assays with Surface Enhanced Raman scattering (SERS) detection to each other and to some other bioassay methods, including ELISA and fluorescence assays. Both immuno- and aptamer-based assays are categorized into assay on solid substrates, assays with magnetic nanoparticles and assays in laminar flow or/and strip assays. The best performing and recent examples of assays in each category are described in the text and illustrated in the figures. The average performance, particularly, limit of detection (LOD) for each of those methods reflected in 9 tables of the manuscript and average LODs are calculated and compared. We found out that, on average, there is some advantage in terms of LOD for SERS immunoassays (0.5 pM median LOD of 88 papers) vs SERS aptamer-based assays (1.7 pM median LOD of 51 papers). We also tabulated and analyzed the clinical performance of SERS immune and aptamer assays, where selectivity, specificity, and accuracy are reported, we summarized the best examples. We also reviewed challenges to SERS bioassay performance and real-life application, including non-specific protein binding, nanoparticle aggregation, limited nanotag stability, sometimes, relatively long time to results, etc. The proposed solutions to those challenges are also discussed in the review. Overall, this review may be interesting not only to bioanalytical chemist, but to medical and life science researchers who are interested in improvement of bioanalyte detection and diagnostics.

A New Detection Method of Oral and Oropharyngeal Squamous Cell Carcinoma Based on Multivariate Analysis of Surface Enhanced Raman Spectra of Salivary Exosomes

Raman spectroscopy recently proved a tremendous capacity to identify disease-specific markers in various (bio)samples being a non-invasive, rapid, and reliable method for cancer detection. In this study, we first aimed to record vibrational spectra of salivary exosomes isolated from oral and oropharyngeal squamous cell carcinoma patients and healthy controls using surface enhancement Raman spectroscopy (SERS). Then, we assessed this method's capacity to discriminate between malignant and non-malignant samples by means of principal component-linear discriminant analysis (PC-LDA) and we used area under the receiver operating characteristics with illustration as the area under the curve to measure the power of salivary exosomes SERS spectra analysis to identify cancer presence. The vibrational spectra were collected on a solid plasmonic substrate developed in our group, synthesized using tangential flow filtered and concentrated silver nanoparticles, capable of generating very reproducible spectra for a whole range of bioanalytes. SERS examination identified interesting variations in the vibrational bands assigned to thiocyanate, proteins, and nucleic acids between the saliva of cancer and control groups. Chemometric analysis indicated discrimination sensitivity between the two groups up to 79.3%. The sensitivity is influenced by the spectral interval used for the multivariate analysis, being lower (75.9%) when the full-range spectra were used.

Recent Trends in SERS-Based Plasmonic Sensors for Disease Diagnostics, Biomolecules Detection, and Machine Learning Techniques

Surface-enhanced Raman spectroscopy/scattering (SERS) has evolved into a popular tool for applications in biology and medicine owing to its ease-of-use, non-destructive, and label-free approach. Advances in plasmonics and instrumentation have enabled the realization of SERS's full potential for the trace detection of biomolecules, disease diagnostics, and monitoring. We provide a brief review on the recent developments in the SERS technique for biosensing applications, with a particular focus on machine learning techniques used for the same. Initially, the article discusses the need for plasmonic sensors in biology and the advantage of SERS over existing techniques. In the later sections, the applications are organized as SERS-based biosensing for disease diagnosis focusing on cancer identification and respiratory diseases, including the recent SARS-CoV-2 detection. We then discuss progress in sensing microorganisms, such as bacteria, with a particular focus on plasmonic sensors for detecting biohazardous materials in view of homeland security. At the end of the article, we focus on machine learning techniques for the (a) identification, (b) classification, and (c) quantification in SERS for biology applications. The review covers the work from 2010 onwards, and the language is simplified to suit the needs of the interdisciplinary audience.

Nanomaterials meet surface-enhanced Raman scattering towards enhanced clinical diagnosis: a review

Surface-enhanced Raman scattering (SERS) is a very promising tool for the direct detection of biomarkers for the diagnosis of i.e., cancer and pathogens. Yet, current SERS strategies are hampered by non-specific interactions with co-existing substances in the biological matrices and the difficulties of obtaining molecular fingerprint information from the complex vibrational spectrum. Raman signal enhancement is necessary, along with convenient surface modification and machine-based learning to address the former issues. This review aims to describe recent advances and prospects in SERS-based approaches for cancer and pathogens diagnosis. First, direct SERS strategies for key biomarker sensing, including the use of substrates such as plasmonic, semiconductor structures, and 3D order nanostructures for signal enhancement will be discussed. Secondly, we will illustrate recent advances for indirect diagnosis using active nanomaterials, Raman reporters, and specific capture elements as SERS tags. Thirdly, critical challenges for translating the potential of the SERS sensing techniques into clinical applications via machine learning and portable instrumentation will be described. The unique nature and integrated sensing capabilities of SERS provide great promise for early cancer diagnosis or fast pathogens detection, reducing sanitary costs but most importantly allowing disease prevention and decreasing mortality rates.

Prognostic prospect of soluble programmed cell death ligand-1 in cancer management

Aggressive tissue biopsy is commonly unavoidable in the management of most suspected tumor cases to conclusively verify the presence of cancerous cells through histological assessment. The extracted tissue is also immunostained for detection of antigens (tissue tumor markers) of potential prognostic or therapeutic importance to assist in treatment decision. Although liquid biopsies can be a powerful tool for monitoring treatment response, they are still excluded from standard cancer diagnostics, and their utility is still being debated in the scientific community. With a myriad of soluble tissue tumor markers now being discovered, liquid biopsies could completely change the current paradigms of cancer management. Recently, soluble programmed cell death ligand-1 (sPD-L1), which is found in the peripheral blood, i.e. serum and plasma, has shown potential as a pre-therapeutic predictive marker as well as a prognostic biomarker to monitor treatment efficacy. Thus, this review focuses on the emergence of sPD-L1 and promising technologies for its detection in order to support liquid biopsies for future cancer management.

Comparison of surface enhanced Raman spectroscopy and Raman spectroscopy for the detection of breast cancer based on serum samples

In this study, surface enhanced Raman spectroscopy (SERS) and Raman spectroscopy (RS), are employed for the classification of different stages of breast cancer using clinically diagnosed serum samples from breast cancer patients and healthy individuals. These serum samples are compared for their spectral features acquired by SERS and RS to establish spectral features that can be considered as spectral markers of breast cancer diagnosis and classification. SERS features related to DNA, proteins and lipids were observed which are solely observed in the serum samples of patients at different stages of breast cancer as compared to healthy samples. In order to explore the capability of SERS and RS and their comparison as an analytical tool for the efficient understanding of the progression of breast cancer, Principal Component Analysis (PCA) is done for the SERS and RS spectra of control, stage 2, stage 3 and stage 4. Furthermore, the Partial Least Squares-Discriminant Analysis (PLS-DA) was performed to compare the diagnostic performance of SERS and Raman spectroscopy for the classification of disease positive samples and healthy ones. The sensitivity and specificity and area under receiver operating characteristic (AUROC) curve values for SERS data were 90%, 98.4%, and 94% respectively which were higher as compared to Raman spectral data for which these values were found to be 88.2%, 97.7%, and 83.4% respectively.

Surface-enhanced Raman scattering-active photonic crystal fiber probe: Towards next generation liquid biopsy sensor with ultra high sensitivity

The tremendous enhancement factors that surface-enhanced Raman scattering (SERS) possesses coupled with the flexibility of photonic crystal fibers (PCFs) pave the way to a new generation of ultrasensitive biosensors. Thanks to the unique structure of PCFs, which allows direct incorporation of an analyte into the axially aligned air channels, interaction between the analyte and excitation light could be increased many folds leading to flexible, reliable and sensitive probes that can be used in preclinical or clinical biosensing. SERS-active PCF probes provide unique opportunity to develop an opto-fluidic liquid biopsy needle sensor that enables one-step integrated sample collection and testing for disease diagnosis. Specificity being a key parameter to biosensors, the PCF inside the biopsy needle could be functionalized with targeting moieties to detect specific biomarkers. In this review article, we present some of the most promising recent biosensors based on PCFs including hollow-core PCFs, suspended-core PCFs and side-channel PCFs. We provide a wide range of applications of such platform using Raman spectroscopy, label free SERS or labeled SERS detection and analyze some of the main challenges to be addressed for translating it to a clinically viable next generation sensitive biopsy needle sensing probe.

Generating Localized Plasmonic Fields on an Integrated Photonic Platform using Tapered Couplers for Biosensing Applications

A theoretical design and analysis of a tapered-coupler structure on a silicon nitride integrated-photonic platform for coupling optical energy from a dielectric waveguide to a plasmonic tip is presented. The proposed design can be considered as a hybrid photonic-plasmonic structure that generally supports hybrid symmetric and asymmetric modes. Along the taper, one of the hybrid modes approaches the cut-off, while the other approaches the short-range surface plasmon mode that generates localized fields. Potential use of the proposed novel tapered-coupler plasmonic structure for highly sensitive biosensing applications using surface enhanced Raman scattering (SERS) and metal enhanced fluorescence (MEF) techniques is discussed. For SERS, a theoretical electromagnetic enhancement factor as high as 1.23 × 10⁶ is deduced for taper tip widths as small as 20 nm. The proposed tapered-coupler sets up interesting possibilities towards moving to an all-integrated on-chip SERS and MEF based bio-sensor platform - away from traditional free-space based illumination strategies.

Formation Regularities of Plasmonic Silver Nanostructures on Porous Silicon for Effective Surface-Enhanced Raman Scattering

Plasmonic nanostructures demonstrating an activity in the surface-enhanced Raman scattering (SERS) spectroscopy have been fabricated by an immersion deposition of silver nanoparticles from silver salt solution on mesoporous silicon (meso-PS). The SERS signal intensity has been found to follow the periodical repacking of the silver nanoparticles, which grow according to the Volmer-Weber mechanism. The ratio of silver salt concentration and immersion time substantially manages the SERS intensity. It has been established that optimal conditions of nanostructured silver layers formation for a maximal Raman enhancement can be chosen taking into account a special parameter called effective time: a product of the silver salt concentration on the immersion deposition time. The detection limit for porphyrin molecules CuTMPyP4 adsorbed on the silvered PS has been evaluated as 10(-11) M.

Prospects for point-of-care pathogen diagnostics using surface-enhanced Raman scattering (SERS)

This review highlights recent advances in the application of surface-enhanced Raman scattering (SERS) in pathogen detection and discusses many of the challenges in moving this technology to the point-of-care (POC) arena.

Magnetic and noble metal nanocomposites for separation and optical detection of biological species

Nanoalloys and nanocomposites are widely studied classes of nanomaterials within the context of biological systems. They are of immense interest because of the possibility of tuning the optical, magnetic, electronic and chemical properties through particle composition and internal architecture. In principle these properties can therefore be optimized for application in biological detections such as of DNA sequences, bacteria, viruses, antibodies, antigens, and cancer cells. This article presents an overview of methods currently used for nanoalloy and nanocomposite synthesis and characterisation, focusing on Au-Ag and FexOy@Au structures as primary components in detection platforms for plasmonic and magnetically enabled plasmonic bio-sensing.

Surface-enhanced Raman scattering on silvered porous alumina templates: role of multipolar surface plasmon resonant modes

Nanostructured silver films with different thicknesses were prepared by vapor deposition onto the surface of the anodic aluminum oxide (AAO) template to be used as surface-enhanced Raman scattering (SERS) active substrates.

Sensitive multiplex detection of serological liver cancer biomarkers using SERS-active photonic crystal fiber probe

Surface-enhanced Raman scattering (SERS) spectroscopy possesses the most promising advantage of multiplex detection for biosensing applications, which is achieved due to the narrow 'fingerprint' Raman spectra from the analyte molecules. We developed an ultrasensitive platform for the multiplex detection of cancer biomarkers by combining the SERS technique with a hollow-core photonic crystal fiber (HCPCF). Axially aligned air channels inside the HCPCF provide an excellent platform for optical sensing using SERS. In addition to the flexibility of optical fibers, HCPCF provides better light confinement and a larger interaction length for the guided light and the analyte, resulting in an improvement in sensitivity to detect low concentrations of bioanalytes in extremely low sample volumes. Herein, for the first time, we demonstrate the sensitive multiplex detection of biomarkers immobilized inside the HCPCF using antibody-conjugated SERS-active nanoparticles (SERS nanotags). As a proof-of-concept for targeted multiplex detection, initially we carried out the sensing of epidermal growth factor receptor (EGFR) biomarker in oral squamous carcinoma cell lysate using three different SERS nanotags. Subsequently, we also achieved simultaneous detection of hepatocellular carcinoma (HCC) biomarkers-alpha fetoprotein (AFP) and alpha-1-antitrypsin (A1AT) secreted in the supernatant from Hep3b cancer cell line. Using a SERS-HCPCF sensing platform, we could successfully demonstrate the multiplex detection in an extremely low sample volume of ∼20 nL. In future, this study may lead to sensitive biosensing platform for the low concentration detection of biomarkers in an extremely low sample volume of body fluids to achieve early diagnosis of multiple diseases. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

Vibrational spectroscopy of biofluids for disease screening or diagnosis: translation from the laboratory to a clinical setting

There remains a need for objective and cost-effective approaches capable of diagnosing early-stage disease in point-of-care clinical settings. Given an increasingly ageing population resulting in a rising prevalence of chronic diseases, the need for screening to facilitate the personalising of therapies to prevent or slow down pathology development will increase. Such a tool needs to be robust but simple enough to be implemented into clinical practice. There is interest in extracting biomarkers from biofluids (e.g., plasma or serum); techniques based on vibrational spectroscopy provide an option. Sample preparation is minimal, techniques involved are relatively low-cost, and data frameworks are available. This review explores the evidence supporting the applicability of vibrational spectroscopy to generate spectral biomarkers of disease in biofluids. We extend the inter-disciplinary nature of this approach to hypothesise a microfluidic platform that could allow such measurements. With an appropriate lightsource, such engineering could revolutionize screening in the 21(st) century.

Emerging advances in nanomedicine with engineered gold nanostructures

Gold nanostructures possess unique characteristics that enable their use as contrast agents, as therapeutic entities, and as scaffolds to adhere functional molecules, therapeutic cargo, and targeting ligands. Due to their ease of synthesis, straightforward surface functionalization, and non-toxicity, gold nanostructures have emerged as powerful nanoagents for cancer detection and treatment. This comprehensive review summarizes the progress made in nanomedicine with gold nanostructures (1) as probes for various bioimaging techniques including dark-field, one-photon and two-photon fluorescence, photothermal optical coherence tomography, photoacoustic tomography, positron emission tomography, and surface-enhanced Raman scattering based imaging, (2) as therapeutic components for photothermal therapy, gene and drug delivery, and radiofrequency ablation, and (3) as a theranostic platform to simultaneously achieve both cancer detection and treatment. Distinct from other published reviews, this article also discusses the recent advances of gold nanostructures as contrast agents and therapeutic actuators for inflammatory diseases including atherosclerotic plaque and arthritis. For each of the topics discussed above, the fundamental principles and progress made in the past five years are discussed. The review concludes with a detailed future outlook discussing the challenges in using gold nanostructures, cellular trafficking, and translational considerations that are imperative for rapid clinical viability of plasmonic nanostructures, as well as the significance of emerging technologies such as Fano resonant gold nanostructures in nanomedicine.

Actively targeted in vivo multiplex detection of intrinsic cancer biomarkers using biocompatible SERS nanotags

Surface-enhanced Raman scattering (SERS) technique is becoming highly popular for multiplex biosensing due to the ‘fingerprint’ Raman spectra from every molecule. As a proof-of-concept, we demonstrated the actively targeted multiplex in vitro and in vivo detection of three intrinsic cancer biomarkers - EGFR, CD44 and TGFβRII in a breast cancer model using three multiplexing capable, biocompatible SERS nanoparticles/nanotags. Intra-tumorally injected antibody conjugated nanotags specifically targeting the three biomarkers exhibited maximum signal at 6 hours and no detectable signal at 72 hours. However, nanotags without antibodies showed no detectable signal after 6 hours. This difference could be due to the specific binding of the bioconjugated nanotags to the receptors on the cell surface. Thus, this study establishes SERS nanotags as an ultrasensitive nanoprobe for the multiplex detection of biomarkers and opens up its potential application in monitoring tumor progression and therapy and development into a theranostic probe.

Using well-defined Ag nanocubes as substrates to quantify the spatial resolution and penetration depth of surface-enhanced Raman scattering imaging

The multiplexing capability and high sensitivity of surface-enhanced Raman scattering (SERS) make this new imaging modality particularly attractive for rapid diagnosis. With 100 nm Ag nanocubes serving as the substrate, this work quantitatively evaluated, for the first time, some of the fundamental parameters of SERS imaging such as blur, spatial resolution and penetration depth. Our results imply that SERS is a high-resolution imaging technique with a blur value of 0.5 μm that is lower than many traditional modalities such as mammography. The spatial resolution was measured to be 1.1 μm, suggesting that SERS images could be collected effectively by adjusting the imaging step size to the same length scale, or no more than 2 μm. The major drawback of SERS imaging is its penetration depth, which is limited by the scattering and absorption of tissues. We demonstrated that enhancement of signal caused by aggregation of multiple nanoparticles could help overcome this potential road-block to in vivo imaging.

Molecular pathology via IR and Raman spectral imaging

During the last 15 years, vibrational spectroscopic methods have been developed that can be viewed as molecular pathology methods that depend on sampling the entire genome, proteome and metabolome of cells and tissues, rather than probing for the presence of selected markers. First, this review introduces the background and fundamentals of the spectroscopies underlying the new methodologies, namely infrared and Raman spectroscopy. Then, results are presented in the context of spectral histopathology of tissues for detection of metastases in lymph nodes, squamous cell carcinoma, adenocarcinomas, brain tumors and brain metastases. Results from spectral cytopathology of cells are discussed for screening of oral and cervical mucosa, and circulating tumor cells. It is concluded that infrared and Raman spectroscopy can complement histopathology and reveal information that is available in classical methods only by costly and time-consuming steps such as immunohistochemistry, polymerase chain reaction or gene arrays. Due to the inherent sensitivity toward changes in the bio-molecular composition of different cell and tissue types, vibrational spectroscopy can even provide information that is in some cases superior to that of any one of the conventional techniques.

Synthesis and characterization of surface-enhanced Raman-scattered gold nanoparticles

In this paper, we report a simple, rapid, and robust method to synthesize surface-enhanced Raman-scattered gold nanoparticles (GNPs) based on green chemistry. Vitis vinifera L. extract was used to synthesize noncytotoxic Raman-active GNPs. These GNPs were characterized by ultraviolet-visible spectroscopy, dynamic light-scattering, Fourier-transform infrared (FTIR), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy. The characteristic surface plasmon-resonance band at ~ 528 nm is indicative of spherical particles, and this was confirmed by TEM. The N-H and C-O stretches in FTIR spectroscopy indicated the presence of protein molecules. The predominant XRD plane at (111) and (200) indicated the crystalline nature and purity of GNPs. GNPs were stable in the buffers used for biological studies, and exhibited no cytotoxicity in noncancerous MIO-M1 (Müller glial) and MDA-MB-453 (breast cancer) cell lines. The GNPs exhibited Raman spectral peaks at 570, 788, and 1,102 cm(-1). These new GNPs have potential applications in cancer diagnosis, therapy, and ultrasensitive biomarker detection.

The development of surface-enhanced Raman scattering as a detection modality for portable in vitro diagnostics: progress and challenges

This perspective provides an overview of the diverse surface-enhanced Raman scattering (SERS)-based sensor platforms that have been developed for in vitro diagnostic applications. To provide focus, protein and nucleic acid detection assays based on the principle of extrinsic SERS sensing are emphasized, as well as their potential for translation to fully integrated point-of-care (POC) test platforms. The development of intrinsic SERS sensors, which are predicated on the direct detection of analytes by laser excitation, entails unique opportunities and challenges deserving of their own attention. As the robust sensing of disease pathogens and cancers in both clinical facilities and limited resource settings is the targeted objective of many next-generation biosensors, the majority of the research progress summarized here centers on SERS sensors developed for the rapid, sensitive and selective detection of disease-causing pathogens and biomarkers. In our effort to communicate a realistic assessment of the progress that has been made and the challenges that lie ahead, we avoid an overtly optimistic appraisal of the current status of SERS diagnostics that does not tacitly acknowledge the difficulties inherent in aligning SERS-based technologies alongside ELISA and PCR technologies as a complementary method for bioanalyte detection possessing unique advantages.

Sensitive SERS-pH sensing in biological media using metal carbonyl functionalized planar substrates

Conventional nanoparticle based Surface enhanced Raman scattering (SERS) technique for pH sensing often fails due to the aggregation of particles when detecting in acidic medium or biosamples having high ionic strength. Here, We develop SERS based pH sensing using a novel Raman reporter, arene chromium tricarbonyl linked aminothiophenol (Cr(CO)3-ATP), functionalized onto a nano-roughened planar substrates coated with gold. Unlike the SERS spectrum of the ATP molecule that dominates in the 400-1700 cm(-1) region, which is highly interfered by bio-molecules signals, metal carbonyl-ATP (Cr(CO)3)-ATP) offers the advantage of monitoring the pH dependent strong CO stretching vibrations in the mid-IR (1800-2200 cm(-1)) range. Raman signal of the CO stretching vibrations at ~1820 cm(-1) has strong dependency on the pH value of the environment, where its peak undergo noticeable shift as the pH of the medium is varied from 3.0 to 9.0. The sensor showed better sensitivity in the acidic range of the pH. We also demonstrate the pH sensing in a urine sample, which has high ionic strength and our data closely correlate to the value obtained from conventional sensor. In future, this study may lead to a sensitive chip based pH sensing platform in bio-fluids for the early diagnosis of diseases.

All-thermoplastic nanoplasmonic microfluidic device for transmission SPR biosensing

Early and accurate disease diagnosis still remains a major challenge in clinical settings. Biomarkers could potentially provide useful tools for the detection and monitoring of disease progression, treatment safety and efficacy. Recent years have witnessed prodigious advancement in biosensor development with research directed towards rapid, real-time, label-free and sensitive biomarker detection. Among emerging techniques, nanoplasmonic biosensors pose tremendous potential to accelerate clinical diagnosis with real-time multiplexed analysis, rapid and miniaturized assays, low sample consumption and high sensitivity. In order to translate these technologies from the proof-of-principle concept level to point of care clinical diagnosis, integrated, portable devices having small footprint cartridges that house low-cost disposable consumables are sought. Towards this goal, we developed an all-polymeric nanoplasmonic microfluidic (NMF) transmission surface plasmon resonance (SPR) biosensor. The device was fabricated in thermoplastics using a simple, single step and cost-effective hot embossing technique amenable to mass production. The novel 3D hierarchical mold fabrication process enabled monolithic integration of blazed nanogratings within the detection chambers of a multichannel microfluidic system. Consequently, a single hard thermoplastic bottom substrate comprising plasmonic and fluidic features allowed integration of active fluidic elements, such as pneumatic valves, in the top soft thermoplastic cover, increasing device functionality. A simple and compact transmission-based optical setup was employed with multiplexed end-point or dual-channel kinetic detection capability which did not require stringent angular accuracy. The sensitivity, specificity and reproducibility of the transmission SPR biosensor was demonstrated through label-free immunodetection of soluble cell-surface glycoprotein sCD44 at clinically relevant picomolar to nanomolar concentrations.

Spectroscopic detection and quantification of heme and heme degradation products

Heme and heme degradation products play critical roles in numerous biological phenomena which until now have only been partially understood. One reason for this is the very low concentrations at which free heme, its complexes and the partly unstable degradation products occur in living cells. Therefore, powerful and specific detection methods are needed. In this contribution, the potential of nondestructive Raman spectroscopy for the detection, quantification and discrimination of heme and heme degradation products is investigated. Resonance Raman spectroscopy using different excitation wavelengths (413, 476, 532, and 752 nm) is employed to estimate the limit of detection for hemin, myoglobin, biliverdin, and bilirubin. Concentrations in the low micromolar range (down to 3 μmol/L) could be reliably detected when utilizing the resonance enhancement effect. Furthermore, a systematic study on the surface-enhanced Raman spectroscopy (SERS) detection of hemin in the presence of other cellular components, such as the highly similar cytochrome c, DNA, and the important antioxidant glutathione, is presented. A microfluidic device was used to reproducibly create a segmented flow of aqueous droplets and oil compartments. Those aqueous droplets acted as model chambers where the analytes have to compete for the colloid. With the help of statistical analysis, it was possible to detect and differentiate the pure substances as well as the binary mixtures and gain insights into their interaction.

Effects of the Excitation Wavelength on the SERS Spectrum

SERS is nowadays a well-established ultrasensitive technique with potential to solve many analytical problems, especially those related to biosciences. This Perspective article aims at summarizing the experimental complexities, in particular, those related to the interaction of light with the sample, that the SERS practitioner may find when acquiring a spectrum while providing a general basis for the interpretation of the obtained vibrational features. With such an idea in mind, factors related to the instrumentation, the optical enhancer, and the analyte molecule are discussed to illustrate the effects of the incident light on the absolute and relative intensity, as well as the spectral profile of the SERS spectra.