New Model for Quantifying the Nanoparticle Concentration Using SERS Supported by Multimodal Mass Spectrometry.

Surface-enhanced Raman scattering (SERS) is widely explored for the elucidation of underlying mechanisms behind biological processes. However, the capability of absolute quantitation of the number of nanoparticles from the SERS response remains a challenge. Here, we show for the first time the development of a new 2D quantitation model to allow calibration of the SERS response against the absolute concentration of SERS nanotags, as characterized by single particle inductively coupled plasma mass spectrometry (spICP-MS). A novel printing approach was adopted to prepare gelatin-based calibration standards containing the SERS nanotags, which consisted of gold nanoparticles and the Raman reporter 1,2-bis(4-pyridyl)ethylene. spICP-MS was used to characterize the Au mass concentration and particle number concentration of the SERS nanotags. Results from laser ablation inductively coupled plasma time-of-flight mass spectrometry imaging at a spatial resolution of 5 µm demonstrated a homogeneous distribution of the nanotags (between-line relative standard deviation < 14%) and a linear response of 197Au with increasing nanotag concentration (R2 = 0.99634) in the printed gelatin standards. The calibration standards were analyzed by SERS mapping, and different data processing approaches were evaluated. The reported calibration model was based on an "active-area" approach, classifying the pixels mapped as "active" or "inactive" and calibrating the SERS response against the total Au concentration and the particle number concentration, as characterized by spICP-MS. This novel calibration model demonstrates the potential for quantitative SERS imaging, with the capability of correlating the nanoparticle concentration to biological responses to further understand the underlying mechanisms of disease models.

Surface Enhanced Raman Spectroscopy for Quantitative Analysis: Results of a Large-Scale European Multi-Instrument Interlaboratory Study.

Surface-enhanced Raman scattering (SERS) is a powerful and sensitive technique for the detection of fingerprint signals of molecules and for the investigation of a series of surface chemical reactions. Many studies introduced quantitative applications of SERS in various fields, and several SERS methods have been implemented for each specific application, ranging in performance characteristics, analytes used, instruments, and analytical matrices. In general, very few methods have been validated according to international guidelines. As a consequence, the application of SERS in highly regulated environments is still considered risky, and the perception of a poorly reproducible and insufficiently robust analytical technique has persistently retarded its routine implementation. Collaborative trials are a type of interlaboratory study (ILS) frequently performed to ascertain the quality of a single analytical method. The idea of an ILS of quantification with SERS arose within the framework of Working Group 1 (WG1) of the EU COST Action BM1401 Raman4Clinics in an effort to overcome the problematic perception of quantitative SERS methods. Here, we report the first interlaboratory SERS study ever conducted, involving 15 laboratories and 44 researchers. In this study, we tried to define a methodology to assess the reproducibility and trueness of a quantitative SERS method and to compare different methods. In our opinion, this is a first important step toward a "standardization" process of SERS protocols, not proposed by a single laboratory but by a larger community.

Correction: Multiplex in vitro detection using SERS.

Correction for 'Multiplex in vitro detection using SERS' by Stacey Laing et al., Chem. Soc. Rev., 2016, 45, 1901-1918.

High Figure of Merit (FOM) of Bragg Modes in Au-Coated Nanodisk Arrays for Plasmonic Sensing.

Gold-coated nanodisk arrays of nearly micron periodicity are reported that have high figure of merit (FOM) and sensitivity necessary for plasmonic refractometric sensing, with the added benefit of suitability for surface-enhanced Raman scattering (SERS), large-scale microfabrication using standard photolithographic techniques and a simple instrumental setup. Gold nanodisk arrays are covered with a gold layer to excite the Bragg modes (BM), which are the propagative surface plasmons localized by the diffraction from the disk array. This generates surface-guided modes, localized as standing waves, leading to highly confined fields confirmed by a mapping of the SERS intensity and numerical simulations with 3D finite element method. The optimal gold-coated nanodisk arrays are applied for refractometric sensing in transmission spectroscopy with better performance than nanohole arrays and they are integrated to a 96-well plate reader for detection of IgY proteins in the nanometer range in PBS. The potential for sensing in biofluids is assessed with IgG detection in 1:1 diluted urine. The structure exhibits a high FOM of up to 46, exceeding the FOM of structures supporting surface plasmon polaritons and comparable to more complex nanostructures, demonstrating that subwavelength features are not necessary for high-performance plasmonic sensing.

A novel nanozyme assay utilising the catalytic activity of silver nanoparticles and SERRS.

Artificial enzymes have become an increasingly interesting area of research due to their many advantages over natural protein enzymes which are expensive, difficult to isolate and unable to stand harsh environments. An important area of this research involves using metal nanoparticles as artificial enzymes, known as nanozymes, which exhibit peroxidase-like activity enabling them to catalyse the oxidation of substrates such as 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H2O2), giving a colorimetric response. Here we exploit the catalytic activity of silver nanoparticles (Ag NPs) in a surface based silver-linked immunosorbent assay (SLISA) to detect human C-reactive protein (CRP), an inflammatory marker. Ag NPs were conjugated to antibodies with specific recognition for the corresponding target antigenic molecule, CRP, and the Ag NPs were used to catalyse the oxidation of TMB by H2O2. The resulting coloured oxidation product was detected using SERRS. We demonstrate that Ag NPs can replace the enzymes used in a conventional ELISA and a detection limit of 1.09 ng mL-1 of CRP can be achieved. It indicates the promise for SLISAs for biomarker detection and opens the way for further assays of this nature to be created. This novel assay has the potential to be optimised to detect lower levels of CRP and can be further extended for the sensitive and specific detection of other relevant biomarkers.

Correction: A novel nanozyme assay utilising the catalytic activity of silver nanoparticles and SERRS.

Correction for 'A novel nanozyme assay utilising the catalytic activity of silver nanoparticles and SERRS' by Sian Sloan-Dennison et al., Analyst, 2017, 142, 2484-2490.

Multiplex in vitro detection using SERS.

The ability to detect multiple disease-related targets from a single biological sample in a quick and reliable manner is of high importance in diagnosing and monitoring disease. The technique known as surface enhanced Raman scattering (SERS) has been developed for the simultaneous detection of multiple targets present in biological samples. Advances in the SERS method have allowed for the sensitive and specific detection of biologically relevant targets, such as DNA and proteins, which could be useful for the detection and control of disease. This review focuses on the strengths of SERS for the detection of target molecules from complex mixtures and the clinical relevance of recent work combining SERS with multiplexed detection of biological targets.

Advancing SERS as a quantitative technique: challenges, considerations, and correlative approaches to aid validation.

Surface-enhanced Raman scattering (SERS) remains a significant area of research since it's discovery 50 years ago. The surface-based technique has been used in a wide variety of fields, most prominently in chemical detection, cellular imaging and medical diagnostics, offering high sensitivity and specificity when probing and quantifying a chosen analyte or monitoring nanoparticle uptake and accumulation. However, despite its promise, SERS is mostly confined to academic laboratories and is not recognised as a gold standard analytical technique. This is due to the variations that are observed in SERS measurements, mainly caused by poorly characterised SERS substrates, lack of universal calibration methods and uncorrelated results. To convince the wider scientific community that SERS should be a routinely used analytical technique, the field is now focusing on methods that will increase the reproducibility of the SERS signals and how to validate the results with more well-established techniques. This review explores the difficulties experienced by SERS users, the methods adopted to reduce variation and suggestions of best practices and strategies that should be adopted if one is to achieve absolute quantification.

Immunoassay arrays fabricated by dip-pen nanolithography with resonance Raman detection.

Here, we report the first use of resonance Raman scattering for the detection of miniaturized microscale arrays fabricated by dip-pen nanolithography. Antibody arrays for prostate-specific antigen (PSA) were printed, and a sandwich immunoassay was carried out. An enzyme-linked detection antibody was used to provide an insoluble and stable colored microdot in the recommended size range for microarray readers, which could be read with resonance Raman scattering. This gives quantitative detection as well as an improved detection limit and a larger dynamic range than that previously achieved by direct fluorescent detection methods. By Raman mapping across the arrayed area, the microdots were easily detected with very little background signal from surrounding areas. Levels of PSA as low as 25 pg/mL were detected using this method, which could be extended to a large number of useful biomarkers.

Tomographic Imaging and Localization of Nanoparticles in Tissue Using Surface-Enhanced Spatially Offset Raman Spectroscopy.

A fundamental question crucial to surface-enhanced spatially offset Raman spectroscopy (SESORS) imaging and implementing it in a clinical setting for in vivo diagnostic purposes is whether a SESORS image can be used to determine the exact location of an object within tissue? To address this question, multiple experimental factors pertaining to the optical setup in imaging experiments using an in-house-built point-collection-based spatially offset Raman spectroscopy (SORS) system were investigated to determine those critical to the three-dimensional (3D) positioning capability of SESORS. Here, we report the effects of the spatial offset magnitude and geometry on locating nanoparticles (NPs) mixed with silica powder as an imaging target through tissue and outline experimental techniques to allow for the correct interpretation of SESORS images to ascertain the correct location of NPs in the two-dimensional x, y-imaging plane at depth. More specifically, the effect of "linear offset-induced image drag" is presented, which refers to a spatial distortion in SESORS images caused by the magnitude and direction of the linear offset and highlight the need for an annular SORS collection geometry during imaging to neutralize these asymmetric effects. Additionally, building on these principles, the concept of "ratiometric SESORS imaging" is introduced for the location of buried inclusions in three dimensions. Together these principles are vital in developing a methodology for the location of surface-enhanced Raman scattering-active inclusions in three dimensions. This approach utilizes the relationship between the magnitude of the spatial offset, the probed depth, and ratiometric analysis of the NP and tissue Raman intensities to ultimately image and spatially discriminate between two distinct NP flavors buried at different depths within a 3D model for the first time. This research demonstrates how to accurately identify multiple objects at depth in tissue and their location using SESORS which addresses a key capability in moving SESORS closer to use in biomedical applications.

Ratiometric imaging of minor groove binders in mammalian cells using Raman microscopy.

Quantitative drug imaging in live cells is a major challenge in drug discovery and development. Many drug screening techniques are performed in solution, and therefore do not consider the impact of the complex cellular environment in their result. As such, important features of drug-cell interactions may be overlooked. In this study, Raman microscopy is used as a powerful technique for semi-quantitative imaging of Strathclyde-minor groove binders (S-MGBs) in mammalian cells under biocompatible imaging conditions. Raman imaging determined the influence of the tail group of two novel minor groove binders (S-MGB-528 and S-MGB-529) in mammalian cell models. These novel S-MGBs contained alkyne moieties which enabled analysis in the cell-silent region of the Raman spectrum. The intracellular uptake concentration, distribution and mechanism were evaluated as a function of the pK a of the tail group, morpholine and amidine, for S-MGB-528 and S-MGB-529, respectively. Although S-MGB-529 had a higher binding affinity to the minor groove of DNA in solution-phase measurements, the Raman imaging data indicated that S-MGB-528 showed a greater degree of intracellular accumulation. Furthermore, using high resolution stimulated Raman scattering (SRS) microscopy, the initial localisation of S-MGB-528 was shown to be in the nucleus before accumulation in the lysosome, which was demonstrated using a multimodal imaging approach. This study highlights the potential of Raman spectroscopy for semi-quantitative drug imaging studies and highlights the importance of imaging techniques to investigate drug-cell interactions, to better inform the drug design process.

Quantitative detection of human tumor necrosis factor α by a resonance raman enzyme-linked immunosorbent assay.

Tumor necrosis factor α is an inflammatory cytokine which has been linked with many infectious and inflammatory diseases. Detection and quantification of this key biomarker is commonly achieved by use of an enzyme-linked immunosorbent assay (ELISA). This fundamental technique uses the spectroscopic detection of a chromogen such as 3,3',5,5'-tetramethylbenzidine (TMB). Horseradish peroxidase (HRP), bound to the detection antibody, catalyzes the oxidation of TMB by hydrogen peroxide to generate colored products which may be measured spectrophotometrically. In this study we have used a conventional ELISA kit and shown that, by replacing the traditional colorimetric detection with resonance Raman spectroscopy, we can achieve 50 times lower detection limits and the potential for multiplexed analysis is increased. In this approach, the laser wavelength was tuned to be in resonance with an electronic transition of the oxidized TMB. The relative intensity of the enhanced Raman bands is proportional to the amount of TMB, thus providing a means of improved quantification. Furthermore, TMB is one of the most widely used chromogenic substrates for HRP-based detection and commercial ELISA test kits, indicating that this detection technique is applicable to a large number of target analytes.

From Raman to SESORRS: moving deeper into cancer detection and treatment monitoring.

Raman spectroscopy is a non-invasive technique that allows specific chemical information to be obtained from various types of sample. The detailed molecular information that is present in Raman spectra permits monitoring of biochemical changes that occur in diseases, such as cancer, and can be used for the early detection and diagnosis of the disease, for monitoring treatment, and to distinguish between cancerous and non-cancerous biological samples. Several techniques have been developed to enhance the capabilities of Raman spectroscopy by improving detection sensitivity, reducing imaging times and increasing the potential applicability for in vivo analysis. The different Raman techniques each have their own advantages that can accommodate the alternative detection formats, allowing the techniques to be applied in several ways for the detection and diagnosis of cancer. This feature article discusses the various forms of Raman spectroscopy, how they have been applied for cancer detection, and the adaptation of the techniques towards their use for in vivo cancer detection and in clinical diagnostics. Despite the advances in Raman spectroscopy, the clinical application of the technique is still limited and certain challenges must be overcome to enable clinical translation. We provide an outlook on the future of the techniques in this area and what we believe is required to allow the potential of Raman spectroscopy to be achieved for clinical cancer diagnostics.

Thermoresponsive Polymer Micropatterns Fabricated by Dip-Pen Nanolithography for a Highly Controllable Substrate with Potential Cellular Applications.

We report a novel approach for patterning thermoresponsive hydrogels based on N,N-diethylacrylamide (DEAAm) and bifunctional Jeffamine ED-600 by dip-pen nanolithography (DPN). The direct writing of micron-sized thermoresponsive polymer spots was achieved with efficient control over feature size. A Jeffamine-based ink prepared through the combination of organic polymers, such as DEAAm, in an inorganic silica network was used to print thermosensitive arrays on a thiol-silanized silicon oxide substrate. The use of a Jeffamine hydrogel, acting as a carrier matrix, allowed a reduction in the evaporation of ink molecules with high volatility, such as DEAAm, and facilitated the transfer of ink from tip to substrate. The thermoresponsive behavior of polymer arrays which swell/deswell in aqueous solution in response to a change in temperature was successfully characterized by atomic force microscopy (AFM) and Raman spectroscopy: a thermally induced change in height and hydration state was observed, respectively. Finally, we demonstrate that cells can adhere to and interact with these dynamic features and exhibit a change in behavior when cultured on the substrates above and below the transition temperature of the Jeffamine/DEAAm thermoresponsive hydrogels. This demonstrates the potential of these micropatterned hydrogels to act as a controllable surface for cell growth.