Dual-Probe SERS Nanosensor: A Promising Approach for Sensitive and Ratiometric Detection of Glucose in Clinical Settings.

Diabetes is a major global health concern, with millions of annual deaths. Monitoring glucose levels is vital for clinical management, and urine samples offer a noninvasive alternative to blood samples. Optical techniques for urine glucose sensing have gained notable traction due to their cost-effectiveness and portability. Among these methods, surface-enhanced Raman spectroscopy (SERS) has attracted considerable attention thanks to its remarkable sensitivity and multiplexing capabilities. However, challenges remain in achieving reliable quantification through SERS. In this study, an alternative approach is proposed to enhance quantification involving the use of dual probes. Each probe is encoded with unique SERS signatures strategically positioned in the biologically silent region. One probe indicates the glucose presence, while the other acts as an internal reference for calibration. This setup enables ratiometric analysis of the SERS signal, directly correlating it with the glucose concentration. The fabrication of the sensor relies on the prefunctionalization of Fe sheets using an aryl diazonium salt bearing a -C≡CH group (internal reference), followed by the immobilization of Ag nanoparticles modified with an aryl diazonium salt bearing a -B(OH)2 group (for glucose capture). A secondary probe bearing a -B(OH)2 group on one side and a -C≡N group on the other side enables the ratiometric analysis by forming a sandwich-like structure in the presence of glucose (glucose indicator). Validation studies in aqueous solutions and artificial urine demonstrated the high spectral stability and the potential of this dual-probe nanosensor for sensitive glucose monitoring in clinical settings.

Recent advances in non-plasmonic surface-enhanced Raman spectroscopy nanostructures for biomedical applications.

Surface-enhanced Raman spectroscopy (SERS) is an emerging powerful vibrational technique offering unprecedented opportunities in biomedical science for the sensitive detection of biomarkers and the imaging and tracking of biological samples. Conventional SERS detection is based on the use of plasmonic substrates (e.g., Au and Ag nanostructures), which exhibit very high enhancement factors (EF = 1010 -1011 ) but suffers from serious limitations, including light-induced local heating effect due to ohmic loss and expensive price. These drawbacks may limit detection accuracy and large-scaled practical applications. In this review, we focus on alternative approaches based on plasmon-free SERS detection on low-cost nanostructures, such as carbons, oxides, chalcogenides, polymers, silicons, and so forth. The mechanism of non-plasmonic SERS detection has been attributed to interfacial charge transfer between the substrate and the adsorbed molecules, with no photothermal side-effects but usually less EF compared with plasmonic nanostructures. The strategies to improve Raman signal detection, through the tailoring of substrate composition, structure, and surface chemistry, is reviewed and discussed. The biomedical applications, for example, SERS cell characterization, biosensing, and bioimaging are also presented, highlighting the importance of substrate surface functionalization to achieve sensitive, accurate analysis, and excellent biocompatibility. This article is categorized under: Diagnostic Tools > Diagnostic Nanodevices Diagnostic Tools > Biosensing Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Correction: Extending nanoscale patterning with multipolar surface plasmon resonances.

Correction for 'Extending nanoscale patterning with multipolar surface plasmon resonances' by Issam Kherbouche et al., Nanoscale, 2021, 13, 11051-11057, DOI: .

New insight into the aptamer conformation and aptamer/protein interaction by surface-enhanced Raman scattering and multivariate statistical analysis.

We study the interaction between one aptamer and its analyte (the MnSOD protein) by the combination of surface-enhanced Raman scattering and multivariate statistical analysis. We observe the aptamer structure and its evolution during the interaction under different experimental conditions (in air or in buffer). Through the spectral treatment by principal component analysis of a large set of SERS data, we were able to probe the aptamer conformations and orientations relative to the surface assuming that the in-plane nucleoside modes are selectively enhanced. We demonstrate that the aptamer orientation and thus its flexibility rely strongly on the presence of a spacer of 15 thymines and on the experimental conditions with the aptamer lying on the surface in air and standing in the buffer. We reveal for the first time that the interaction with MnSOD induces a large loss of flexibility and freezes the aptamer structure in a single conformation.

Extending nanoscale patterning with multipolar surface plasmon resonances.

Plasmonic excitation of metallic nanoparticles can trigger chemical reactions at the nanoscale. Such optical effects can also be employed to selectively and locally graft photopolymer layers at the nanostructure surface, and, when combined with a surface functionalization agent, new pathways can be explored to modify the surface of a plasmonic nanoparticle. Among these approaches, diazonium salt chemistry is seen as an attractive strategy due to the high photoinduced reactivity of these salts. In this work, we demonstrate that it is possible to trigger the site-selective grafting of aryl films derived from diazonium salts on distinct nano-localized area of single gold nanotriangles, by taking advantage of their multipolar localized surface plasmon modes. It is shown the aryl film will preferentially graft in areas where the electric field enhancement is maximum, independently of the considered excited surface plasmon mode. These experimental findings are in very good qualitative agreement with the calculations of the local electric field, using the finite-difference time-domain (FDTD) method. We believe that this plasmonic-based approach will not only pave a new way for the spatially controlled surface functionalization of plasmonic nanoparticles, but also provide a general strategy to attach distinct molecules to hot spot regions on a single nanoparticle, opening promising prospects in sensing and multiplexing, and optically nano-scale patterning of functional groups.

Introducing cobalt as a potential plasmonic candidate combining optical and magnetic functionalities within the same nanostructure.

The control of magnetic properties at the nanoscale is a current topic of intense research. It was shown that combining both magnetic and plasmonic nanoparticles (NPs) led to the improvement of their magneto-optical signal. In this context, common strategies consist of the design of bimetallic NPs. However, the understanding of the physics leading to the coupling between magnetic and plasmonic NPs is lacking, preventing any significant progress for the development of future photonic devices. In this article, we propose to focus our attention on an efficient and commonly used magnetic metal, cobalt, and evaluate its plasmonic properties at the nanoscale through the use of NP regular arrays, as a potential candidate combining both optical and magnetic functionalities within the same metal. We show that such NPs display plasmonic properties within a large spectral range from the UV to the NIR spectral range, with efficient quality factors, when the inter-particle distance is properly selected. These as-fabricated simple materials could find applications in integrated photonic devices for telecommunications.

Three-color plasmon-mediated reduction of diazonium salts over metasurfaces.

Surface plasmon-mediated chemical reactions are of great interest for a variety of applications ranging from micro- and nanoscale device fabrication to chemical reactions of societal interest for hydrogen production or carbon reduction. In this work, a crosshair-like nanostructure is investigated due to its ability to induce local enhancement of the local electromagnetic field at three distinct wavelengths corresponding to three plasmon resonances. The structures are irradiated in the presence of a solution containing diazonium salts at wavelengths that match the resonance positions at 532 nm, 632.8 nm, and 800 nm. The resulting grafting shows polarization and wavelength-dependent growth patterns at the nanoscale. The plasmon-mediated reactions over arrays of the crosshair structures are further investigated using scanning electron microscopy and supported by finite domain time domain modelling revealing wavelength and polarization specific reactions. Such an approach enables nanoscale molecular printing using light source opening multiplexing applications where different analytes can be grafted under distinct opto-geometric conditions.

Surface Enhanced Raman Scattering on Regular Arrays of Gold Nanostructures: Impact of Long-Range Interactions and the Surrounding Medium.

Long-range interaction in regular metallic nanostructure arrays can provide the possibility to manipulate their optical properties, governed by the excitation of localized surface plasmon (LSP) resonances. When assembling the nanoparticles in an array, interactions between nanoparticles can result in a strong electromagnetic coupling for specific grating constants. Such a grating effect leads to narrow LSP peaks due to the emergence of new radiative orders in the plane of the substrate, and thus, an important improvement of the intensity of the local electric field. In this work, we report on the optical study of LSP modes supported by square arrays of gold nanodiscs deposited on an indium tin oxyde (ITO) coated glass substrate, and its impact on the surface enhanced Raman scattering (SERS) of a molecular adsorbate, the mercapto benzoic acid (4-MBA). We estimated the Raman gain of these molecules, by varying the grating constant and the refractive index of the surrounding medium of the superstrate, from an asymmetric medium (air) to a symmetric one (oil). We show that the Raman gain can be improved with one order of magnitude in a symmetric medium compared to SERS experiments in air, by considering the appropriate grating constant. Our experimental results are supported by FDTD calculations, and confirm the importance of the grating effect in the design of SERS substrates.

Enhanced Photochromism of Diarylethene Induced by Excitation of Localized Surface Plasmon Resonance on Regular Arrays of Gold Nanoparticles.

Localized surface plasmon resonance (LSPR) excitation on the photochromic reaction of a diarylethene derivative (DE) was studied by surface enhanced Raman scattering (SERS). UV and visible light irradiations transform reversibly DE between open-form (OF) and closed-form (CF) isomers, respectively. A mixture of PMMA and DE (either OF or CF isomer) was spin-coated onto gold nanorods (GNRs) arrays, designed by electron beam lithography, with two localized surface plasmon resonances (LSPR) at distinct wavelengths, due to their anisotropy. The photochromic reaction rates from CF to OF isomers, under LSPR excitation, were monitored from SERS spectral changes under different polarizations, on the same GNR substrate to compare the effect of LSPR field strength. It appears that the photoisomerization rate was faster when LSPR was excited with the polarization parallel to the GNR long axis. The present results highlight a potential genuine mechanism, from near field LSPR excitation, involved in the photochromic enhancement of diarylethene photochromes.

Rapid and sensitive identification of uropathogenic Escherichia coli using a surface-enhanced-Raman-scattering-based biochip.

Rapid, selective and sensitive sensing of bacteria remains challenging. We report on a highly sensitive and reproducible surface-enhanced Raman spectroscopy (SERS)-based sensing approach for the detection of uropathogenic Escherichia coli (E. coli) bacteria in urine. The assay is based on the specific capture of the bacteria followed by interaction with cetyltrimethylammonium bromide (CTAB)-stabilised gold nanorods (Au NRS) as SERS markers. High sensitivity up to 10 CFU mL-1 is achieved by optimizing the capture interface based on hydrogenated amorphous silicon a-Si:H thin films. The integration of CH3O-PEG750 onto a-Si:H gives the sensing interface an efficient anti-fouling character, while covalent linkage of antibodies directed against the major type-1 fimbrial pilin FimA of the human pathogen E. coli results in the specific trapping of fimbriated E. coli onto the SERS substrate and their spectral fingerprint identification.

Gold Nanocylinders on Gold Film as a Multi-spectral SERS Substrate.

The surface enhanced Raman scattering (SERS) efficiency of gold nanocylinders deposited on gold thin film is studied. Exploiting the specific plasmonic properties of such substrates, we determine the influence of the nanocylinder diameter and the film thickness on the SERS signal at three different excitation wavelengths (532, 638 and 785 nm). We demonstrate that the highest signal is reached for the highest diameter of 250 nm due to coupling between the nanocylinders and for the lowest thickness (20 nm) as the excited plasmon is created at the interface between the gold and glass substrate. Moreover, even if we show that the highest SERS efficiency is obtained for an excitation wavelength of 638 nm, a large SERS signal can be obtained at all excitation wavelengths and on a wide spectral range. We demonstrate that it can be related with the nature of the plasmon (propagative plasmon excited through the nanocylinder grating) and with its angular dependence (tuning of the plasmon position with the excitation angle). Such an effect allows the excitation of plasmon on nearly the whole visible range, and paves the way to multispectral SERS substrates.

Raman reporters derived from aryl diazonium salts for SERS encoded-nanoparticles.

Surface-enhanced Raman scattering (SERS) tags are usually prepared by immobilizing Raman reporters on plasmonic nanoparticles (NPs) via thiol-based self-assembled monolayers. We describe here the first example of SERS tags obtained by combining gold NPs and aryl diazonium salts. This strategy results in robust Au-C covalent bonds between the Raman reporter and the NPs, thus ensuring a high stability of the nanohybrid interface.

Dynamic Plasmonic Platform To Investigate the Correlation between Far-Field Optical Response and SERS Signal of Analytes.

The design of surface-enhanced Raman spectroscopy (SERS) platforms based on the coupling between plasmonic nanostructures and stimuli-responsive polymers has attracted considerable interest over the past decades for the detection of a wide range of analytes, including pollutants and biological molecules. However, the SERS intensity of analytes trapped inside smart hybrid nanoplatforms is subject to important fluctuations because of the spatial and spectral variation of the plasmonic near-field enhancement (i.e., its dependence with the distance to the nanoparticle surface and with the localized surface plasmon resonance). Such fluctuations may impair interpretation and quantification in sensing devices. In this paper, we investigate the influence of the plasmonic near-field profile upon the Raman signal intensity of analytes trapped inside thermoresponsive polymer-coated gold nanoarrays. For this, well-defined plasmonic arrays (nanosquares and nanocylinders) were modified by poly(N-isopropylacrylamide) (PNIPAM) brushes using surface-initiated atom-transfer radical polymerization. Molecular probes were trapped inside these Au@PNIPAM nanostructures by simple physisorption or by covalent grafting at the end of PNIPAM brushes, using click chemistry. The SERS spectra of molecular probes were studied along various heating/cooling cycles, demonstrating a strong correlation between SERS intensities and near-field spectral profile of underlying nanoparticles, as confirmed by simulations based on the finite difference time domain method. Thermoresponsive plasmonic devices thus provide an ideal dynamic SERS platform to investigate the influence of the near-field plasmonic profile upon the SERS response of analytes.

Photochromic control of a plasmon-quantum dots coupled system.

The control of quantum dot (QD) photoluminescence (PL) is a challenge for many applications. It is well known that plasmonic resonances can enhance this PL. In this work, we couple QDs with silver nanoparticles and immerse the system in a photochromic organic material. As these molecules are optical switches going from a transparent to a colored isomer by absorbing UV light, we observe on one hand a Förster Resonant Energy Transfer (FRET) between the QD emission and the absorbing isomer and on the other hand a plasmonic PL enhancement. The photochromic transition leads to the optical control of the FRET, allowing us to control the QD de-excitation preferences (radiative or non-radiative) and so the emitted light.

Polarization-dependent strong coupling between silver nanorods and photochromic molecules.

Active plasmonics is a key focus for the development of advanced plasmonic applications. By selectively exciting the localized surface plasmon resonance sustained by the short or the long axis of silver nanorods, we demonstrate a polarization-dependent strong coupling between the plasmonic resonance and the excited state of photochromic molecules. By varying the width and the length of the nanorods independently, a clear Rabi splitting appears in the dispersion curves of both resonators.

Regioselective surface functionalization of lithographically designed gold nanorods by plasmon-mediated reduction of aryl diazonium salts.

Site-selective surface functionalization of anisotropic gold nanoparticles represents a major breakthrough for fully exploiting nanoparticle anisotropy. In this paper, we explore an original strategy for the regioselective functionalization of lithographically designed gold nanorods (AuNRs), based a combination of photo-induced plasmon excitation and aryl diazonium salt chemistry.

Water-Dispersed Hydrophobic Au Nanocrystal Assemblies with a Plasmon Fingerprint.

Hydrophobic Au nanocrystal assemblies (both ordered and amorphous) were dispersed in aqueous solution via the assistance of lipid vesicles. The intertwine between vesicles and Au assemblies was made possible through a careful selection of the length of alkyl chains on Au nanocrystals. Extinction spectra of Au assemblies showed two peaks that were assigned to a scattering mode that red-shifted with increasing the assembly size and an absorption mode associated with localized surface plasmon that was independent of their size. This plasmon fingerprint could be used as a probe for investigating the optical properties of such assemblies. Our water-soluble assemblies enable exploring a variety of potential applications including solar energy and biomedicine.

Oriented Gold Nanorods and Gold Nanorod Chains within Smectic Liquid Crystal Topological Defects.

We show that the use of oriented linear arrays of smectic A defects, the so-called smectic oily streaks, enables the orientation of gold nanorods (GNRs) for a large range of GNR diameters, ranging from 7 to 48 nm, and for various ligands. For the small GNRs it enables oriented end-to-end small chains of GNRs when the density is increased from around 2 GNRs/µm2 to around 6 GNRs/µm2. We have characterized the orientation of single GNRs by spectrophotometry and two-photon luminescence (TPL). A strongly anisotropic absorption of the composites and an on-off switching of GNR luminescence, both controlled by incident light polarization, are observed, revealing an orientation of the GNRs mostly parallel to the oily streaks. A more favorable trapping of GNRs by smectic dislocations with respect to ribbon-like defects is thus demonstrated. The dislocations appear to be localized at a specific localization, namely, the summit of rotating grain boundaries. Combining plasmonic absorption measurements, TPL measurements, and simulation of the plasmonic absorption, we show that the end-to-end GNR chains are both dimers and trimers, all parallel to each other, with a small gap between the coupled GNRs, on the order of 1.5 nm, thus associated with a large red-shift of 110 nm of the longitudinal plasmonic mode. A motion of the GNRs along the dislocations appears as a necessary ingredient for the formation of end-to-end GNR chains, the gap value being driven by the balance between the attracting van der Waals interactions and the steric repulsion between the GNRs and leading to interdigitation of the neighboring ligands. We thus obtain electromagnetic coupling of nanorods controlled by light polarization.

Highly stable silica-coated gold nanorods dimers for solution-based SERS.

The controlled assembly of anisotropic plasmonic nanoparticles (NPs) into highly SERS-active substrates remains particularly challenging for the production of long-term stable NP assemblies in suspension. In this work, we report a simple and efficient strategy to assemble gold nanorods (AuNRs) into dimers. The pH-dependent assembly was triggered using the bifunctional molecular linker BPE (1,2-bis(4-pyridyl)ethylene) and quenched with silver nitrate. The resulting AuNR dimers were encapsulated in mesoporous silica shell and proved to be stable in water for at least 5 months. Taking advantage of the large Raman scattering cross-section of the linker BPE, we conducted a detailed study of the enhancement ability of these NR dimers using solution-based surface enhanced Raman scattering (SERS). Both experimental (SERS) and theoretical (discrete dipole approximation) studies of the near-field characteristics revealed a two-orders of magnitude increase of the SERS enhancement factor for the dimers as compared to isolated AuNRs. Besides thermal and colloidal stability, mesoporous silica coating of AuNRs imparts other notable advantages due to its porosity and biocompatibility, which make these core-shell plasmonic platforms promising for future bio-applications.