DNA Origami-Based Nanoprinting for the Assembly of Plasmonic Nanostructures with Single-Molecule Surface-Enhanced Raman Scattering

Metallic nanocube ensembles exhibit tunable localized surface plasmon resonance to induce the light manipulation at the subwavelength scale. Nevertheless, precisely control anisotropic metallic nanocube ensembles with relative spatial directionality remains a challenge. Here, we report a DNA origami based nanoprinting (DOBNP) strategy to transfer the essential DNA strands with predefined sequences and positions to the surface of the gold nanocubes (AuNCs). These DNA strands ensured the specific linkages between AuNCs and gold nanoparticles (AuNPs) that generating the stereo-controlled AuNC-AuNP nanostructures (AANs) with controlled geometry and composition. By anchoring the single dye molecule in hot spot regions, the dramatic enhanced electromagnetic field aroused stronger surface enhanced Raman scattering (SERS) signal amplification. Our approach opens the opportunity for the fabrication of stereo-controlled metal nanostructures for designing highly sensitive photonic devices.

Aptamer-Regulated Gold Nanosol Plasmonic SERS/RRS Dimode Assay of Trace Organic Pollutants Based on TpPa-Loaded PdNC Catalytic Amplification

As with excellent catalytic performance, palladium nanoclusters (PdNCs) have a wide range of applications. However, the traditional PdNCs are easy to agglomerate in the analysis system and lose their catalytic activity. A covalent organic framework (COF) has a definite structure, good stability, and easy surface functionalization. So, it is of great significance to develop stable PdNCs with high catalytic activity and then combine with advanced analysis techniques to analyze ultratrace small-molecule pollutants in the environment. In this research, a stable PdNC dispersed on a COF (PdTpPa) catalyst is prepared and we find it with strong catalysis for the NaH2PO2-HAuCl4 catalytic reaction. Furthermore, this nanocatalytic indicator reaction can be tracked by surface-enhanced Raman spectroscopy (SERS) and resonance Rayleigh scattering (RRS) dual-mode. Combined with a highly specific aptamer-modifying technique, a highly sensitive and selective SERS/RRS dimode assay platform for trace organic pollutants has been developed. The detection limits of oxytetracycline (OTC), glyphosate (GLY), tetracycline (TEC), and bisphenol A (BPA) are 0.64, 0.03, 6.2 × 10-3, and 0.53 × 10-3 ng/mL, respectively. This work also provides ideas for the application of COF materials and Pd nanocatalysts in the molecular spectral detection of trace pollutants.

Electrospun polymeric nanofiber decorated with sea urchin-like gold nanoparticles as an efficient and stable SERS platform

Surface enhanced Raman scattering (SERS)-based nanoprobes have been used as well-established analytical tools enabling single-molecule detection. In this work, we report a facile method to decorate sea urchin-like gold nanoparticles (SUGNPs) on the surface of PMMA/P4VP nanofibers. Firstly, PMMA/P4VP nanofibers within the submicrometer size range were prepared by applying the electrospinning technique. Then, the incorporation of SUGNPs on the surface of PMMA/P4VP nanofiber was achieved by immersing PMMA/P4VP nanofiber into freshly prepared SUGNP aqueous solution through the specific Au-N interactions. The as-fabricated SUGNP-coated PMMA/P4VP nanofibers exhibited good sensitivity and reproducibility in SERS measurements with the relative standard deviation down to 6.6%, by employing 4-mercaptobenzoic acid as a probe molecule with 30 min of soaking time. Hence, we envisage that the SUGNP-coated PMMA/P4VP nanofibers can act as efficient and stable SERS substrates for potential applications in molecular detection as well as chemical and biological analysis.

Vibrational spectroscopy and DFT analysis of 4-cyanophenylhydrazine: A potential SERS probe

4-Cyanophenylhydrazine (4-CPH) is an organic synthesis intermediate. To date, several products derived from 4-CPH have been well studied; however, 4-CPH itself has not been extensively investigated. Herein, we performed vibrational and theoretical analyses of 4-CPH. Density functional theory (DFT) calculations were applied to predict the IR and Raman spectra of 4-CPH, which were compared with the experimental spectra. The calculated and experimental spectral results were in good agreement, except for an abnormal transformation of the protonated 4-CPH cyano group (C≡N), which was observed in the theoretical IR spectrum. Several wavefunction analyses revealed that this transformation was due to the protonation-induced depolarization of the molecule. Moreover, we verified the applicability of 4-CPH as a probe for surface-enhanced Raman spectroscopy (SERS). We observed a pH-dependent shift in the cyano bond frequency within the silent region and determined, as a novel discovery, that this shift was induced by 4-CPH protonation. Our results provide considerable, fundamental information that confirms the potential of 4-CPH as a SERS probe.

Charge-Transfer Induced by the Oxygen Vacancy Defects in the Ag/MoO3 Composite System

In this paper, an Ag/MoO3 composite system was cosputtered by Ar plasma bombardment on a polystyrene (PS) colloidal microsphere array. The MoO3 formed by this method contained abundant oxygen vacancy defects, which provided a channel for charge transfer in the system and compensated for the wide band gap of MoO3. Various characterization methods strongly demonstrated the existence of oxygen vacancy defects and detected the properties of oxygen vacancies. 4-Aminothiophenol (p-aminothiophenol, PATP) was used as a candidate surface-enhanced Raman scattering (SERS) probe molecule to evaluate the contribution of the oxygen vacancy defects in the Ag/MoO3 composite system. Interestingly, oxygen vacancy defects are a kind of charge channel, and their powerful effect is fully reflected in their SERS spectra. Increasing the number of charge channels and increasing the utilization rate of the channels caused the frequency of SERS characteristic peaks to shift. This interesting phenomenon opens up a new horizon for the study of SERS in oxygen-containing semiconductors and provides a powerful reference for the study of PATP.

Drug preconcentration and direct quantification in biofluids using 3D-Printed paper cartridge

Drug detection in biofluids has always been great importance for clinical diagnosis. Many detection technologies such as chromatography-mass spectrometry, have been applied to the detection of drugs. However, these technologies required multi-step operations, including complicated and time-consuming pretreatment processes and operations of bulky detection instruments, significantly limiting development of drug detection in clinical diagnosis. Herein, a portable 3D-printed paper cartridge was fabricated for fast sample preconcentration and direct drugs quantitative detection in biofluids by a portable Raman spectrometer. This cartridge contained both paper tip with silver nanowires to preconcentrate samples and achieve surface-enhanced Raman Scattering (SERS) measurement, and 3D-printed cartridge to build enclosed environment for the improvement of detection, which cost only one dollar. The preconcentration ability of the cartridge was up to 18.13-fold fluorescence enhancement and compared to the non-preconcentration method, it achieved 9.93-fold improvement of SERS performance. The anticancer drug of epirubicin hydrochloride, cyclophosphamide and their mixtures were quantitatively detected in the bovine serum or artificial urine. The integrated detection procedure required only 1 h, including sample pretreatment and preconcentration, drying, SERS measurements, and quantification analysis. This 3D-printed paper cartridge constituted a portable detection platform that would be potentially a practical and point-of-care detection tool for drug preconcentration and quantification on the clinical diagnosis.

Three-dimensional SERS sensor based on the sandwiched G@AgNPs@G/PDMS film

Three-dimensional (3D) SERS substrate with the denser "hotspots" is synthesized by the constriction of PDMS film decorated with sandwiched graphene@AgNPs@graphene (G@AgNPs@G) nanostructure. Graphene layers above and below the AgNPs are used to absorb molecules onto the "hotspots", and prevent the oxidation of AgNPs in our design. PDMS films can be easily shrunk for 3D structures, causing advantages in enhancement ability and light-matter interaction. Benefiting from the above advantages, a detection limit of 10^-14 M (CV) and enhancement factor (EF) of 3.9 × 10⁹ were obtained in our experiment. Theoretical analyses (FDTD) were also used to study the enhancement mechanism. For practical purposes, in-situ detection of MG molecules on the fish surface and the label-free detection of DNA base of adenine (A) and cytosine (C) were also studied. The high enhancement factor, great sensitivity, reliability, and stability of substrate reasonably proved that it can be used as an excellent SERS substrate for biomolecular detection.

Surface-enhanced Raman spectroscopy analysis of serum samples of typhoid patients of different stages

BACKGROUND

Surface-enhanced Raman spectroscopy (SERS) of body fluids is considered a quick, simple and easy to use method for the diagnosis of disease.

OBJECTIVES

To evaluate rapid, reliable, and non-destructive SERS-based diagnostic tool with multivariate data analysis including principal component analysis (PCA) and partial least square discriminant analysis (PLS-DA) for classification of different stages of typhoid on the basis of characteristic SERS spectral features.

METHODS

SERS has been used for analysis of serum samples of different stages of typhoid including early acute stage and late acute stage in comparison with healthy samples, in order to investigate capability of this technique for diagnosis of typhoid. SERS spectral features associated with the biochemical changes taking place during the development of the typhoid fever were analyzed and identified.

RESULTS

The value of area under the receiver operating characteristics (AUROC) for early acute stage versus healthy is 0.87 and that for healthy versus late acute stage is 0.52. PLS-DA classifier model gives values of 100 % for accuracy, sensitivity and specificity, respectively for the SERS spectral data sets of healthy versus early acute stage. Moreover, this classifier model gives values of 91 %, 89 % and 97 % for accuracy, sensitivity and specificity, respectively for the SERS spectral data sets of healthy versus late acute stage.

CONCLUSIONS

Based on preliminary work it is concluded that SERS has potential to diagnose various stages of typhoid fever including early acute and late acute stage in comparison with healthy samples.

Detection of multi-class pesticide residues with surface-enhanced Raman spectroscopy

The excessive use of pesticides disturbs the natural balance in the environment, creates resistance to pesticides and leads to water and food contamination. Therefore, the implementation of fast, robust and cost effective techniques for the monitoring of pesticides is required. In this work surface-enhanced Raman spectroscopy (SERS) was used for the detection of four common pesticides: atrazine, simazin, irgarol, and diuron. SERS is nowadays considered an effective technique for detection of various analytes in low concentration. Sensitivity of the SERS method depends on the type of substrate that can be either a colloidal solution of metal nanoparticles (NPs) or a metal surface with a suitable nanostructured topology. Here, we have investigated the application of silver nanospheres and silver nanoprisms as SERS substrates in pesticides detection. Colloids with spherical NPs were produced by chemical reduction while Ag nanoprisms were prepared by reducing silver nitrate with borohydride (with citrate as a stabilizing agent) and stirring under a UV lamp for 4 and 10 h. The SERS results have shown that, in the presence of synthesized NPs, it was possible to detect millimolar concentrations of aforementioned pesticides with the exception of diuron.

Spectroscopy of model-membrane liposome-protein systems: complementarity of linear dichroism, circular dichroism, fluorescence and SERS

A range of membrane models have been developed to study components of cellular systems. Lipid vesicles or liposomes are one such artificial membrane model which mimics many properties of the biological system: they are lipid bilayers composed of one or more lipids to which other molecules can associate. Liposomes are thus ideal to study the roles of cellular lipids and their interactions with other membrane components to understand a wide range of cellular processes including membrane disruption, membrane transport and catalytic activity. Although liposomes are much simpler than cellular membranes, they are still challenging to study and a variety of complementary techniques are needed. In this review article, we consider several currently used analytical methods for spectroscopic measurements of unilamellar liposomes and their interaction with proteins and peptides. Among the variety of spectroscopic techniques seeing increasing application, we have chosen to discuss: fluorescence based techniques such as FRET (fluorescence resonance energy transfer) and FRAP (fluorescence recovery after photobleaching), that are used to identify localisation and dynamics of molecules in the membrane; circular dichroism (CD) and linear dichroism (LD) for conformational and orientation changes of proteins on membrane binding; and SERS (Surface Enhanced Raman Spectroscopy) as a rapidly developing ultrasensitive technique for site-selective molecular characterisation. The review contains brief theoretical basics of the listed techniques and recent examples of their successful applications for membrane studies.

A Sensitive Surface-Enhanced Raman Spectroscopy Method for Detecting Tetracycline in Milk

Tetracycline, an animal antibiotic, may remain in milk to cause harm to human health. For economic reasons, the abuse of antibiotics is becoming more and more common. Therefore, the abuse of tetracycline has alarmed the dairy industry and many countries such as New Zealand, China, and the USA have proposed strict standards. Surface-enhanced Raman scattering (SERS) is an emerging detection method which has been applied in food detection with the advantages of no complex pretreatment, fast detection, and weak water environment interference. Considering the abuse of antibiotics in dairy industry, we used polydimethylsiloxane (PDMS) plasma cavity as SERS substrate to detect tetracycline in milk. We found that the enhancement ability of PDMS substrate is affected by addition of 4-amino-1-butanol and complex interplay in the milk--tetracycline system. The modified PDMS plasma cavity has high SERS sensitivity that allows us to achieve low detection limit of 0.28 μg/L. The correlation coefficient was 0.987. The detection of tetracycline in milk using PDMS substrate is quick (within 10 min) and it provides a possible method for in-site detection of tetracycline.

Prospects in interfaces of biomolecule DNA and nanomaterials as an effective way for improvising surface enhanced Raman scattering: A review

Surface Enhanced Raman Scattering (SERS) is a field of research that has shown promising application in the analysis of various substrate molecules by means of rough metallic surfaces. In directing the enhancement of substrate molecules in micro and nano-molar concentrations, plasmonic coupling of metal nanoparticles (NPs), morphology of metal NPs and the closely arrangement of rough metal surfaces that produces 'hot spots' can effectively increase the so-called enhancement factor (EF) that will be applicable in various fields. As the mechanistic aspects are still not clear, research has been triggered all over the world for the past two decades to have a clear understanding in chemical and electromagnetic effects. As the reproducibility of intensity of signals at low concentrations of probe molecules is of a big concern, metal NPs with various scaffolds were prepared and recently bio-molecule, DNA has been studied and showed promising advantages. This review first time highlights metal NPs with DNA interface as an effective rough metallic surface for SERS with high intensity and also with better reproducibility. Based on this review, similar kinds of scaffolds like DNA can be used to further analyze SERS activities of various metal NPs with different morphologies to have high intense signals at low concentrations of probe molecules.

Probing the Intracellular Bio-Nano Interface in Different Cell Lines with Gold Nanostars

Gold nanostars are a versatile plasmonic nanomaterial with many applications in bioanalysis. Their interactions with animal cells of three different cell lines are studied here at the molecular and ultrastructural level at an early stage of endolysosomal processing. Using the gold nanostars themselves as substrate for surface-enhanced Raman scattering, their protein corona and the molecules in the endolysosomal environment were characterized. Localization, morphology, and size of the nanostar aggregates in the endolysosomal compartment of the cells were probed by cryo soft-X-ray nanotomography. The processing of the nanostars by macrophages of cell line J774 differed greatly from that in the fibroblast cell line 3T3 and in the epithelial cell line HCT-116, and the structure and composition of the biomolecular corona was found to resemble that of spherical gold nanoparticles in the same cells. Data obtained with gold nanostars of varied morphology indicate that the biomolecular interactions at the surface in vivo are influenced by the spike length, with increased interaction with hydrophobic groups of proteins and lipids for longer spike lengths, and independent of the cell line. The results will support optimized nanostar synthesis and delivery for sensing, imaging, and theranostics.

Widely tunable surface plasmon resonance and uniquely superior SERS performance of Au nanotube network films

Three-dimensional Au network films with flexibility and transferability were fabricated based on sputtering deposition onto electrospun nanofibers as a template. The films are constructed using long Au nanotubes that are cross-linked with each other and that have dense nanoparticles on the tube wall surface. The surface plasmon resonance (SPR) peaks for the films are tunable in a wide range, from visible light to the near-infrared region, by tuning the inner diameter and/or wall thickness of the nanotubes. Such structured film exhibits significant surface-enhanced Raman scattering (SERS) activity with good signal uniformity and stability, and possesses great potential in thein situdetection of trace organic pollutants on a solid surface by simple transferring. This study provides a Au film with a unique structure and widely tunable SPR forin situSERS sensing and other needs.

Detection of Sub-Micro- and Nanoplastic Particles on Gold Nanoparticle-Based Substrates through Surface-Enhanced Raman Scattering (SERS) Spectroscopy

Small plastic particles such as micro- (<5 mm), sub-micro- (1 µm-100 nm) and nanoplastics (<100 nm) are known to be ubiquitous within our surrounding environment. However, to date relatively few methods exist for the reliable detection of nanoplastic particles in relevant sample matrices such as foods or environmental samples. This lack of relevant data is likely a result of key limitations (e.g., resolution and/or scattering efficiency) for common analytical techniques such as Fourier transform infrared or Raman spectroscopy. This study aims to address this knowledge gap in the field through the creation of surface-enhanced Raman scattering spectroscopy substrates utilizing spherical gold nanoparticles with 14 nm and 46 nm diameters to improve the scattering signal obtained during Raman spectroscopy measurements. The substrates are then used to analyze polystyrene particles with sizes of 161 nm or 33 nm and poly(ethylene terephthalate) particles with an average size of 62 nm. Through this technique, plastic particles could be detected at concentrations as low as 10 µg/mL, and analytical enhancement factors of up to 446 were achieved.

Nanoporous Metals: From Plasmonic Properties to Applications in Enhanced Spectroscopy and Photocatalysis

The field of plasmonics is capable of enabling interesting applications in different wavelength ranges, spanning from the ultraviolet up to the infrared. The choice of plasmonic material and how the material is nanostructured has significant implications for ultimate performance of any plasmonic device. Artificially designed nanoporous metals (NPMs) have interesting material properties including large specific surface area, distinctive optical properties, high electrical conductivity, and reduced stiffness, implying their potentials for many applications. This paper reviews the wide range of available nanoporous metals (such as Au, Ag, Cu, Al, Mg, and Pt), mainly focusing on their properties as plasmonic materials. While extensive reports on the use and characterization of NPMs exist, a detailed discussion on their connection with surface plasmons and enhanced spectroscopies as well as photocatalysis is missing. Here, we report on different metals investigated, from the most used nanoporous gold to mixed metal compounds, and discuss each of these plasmonic materials' suitability for a range of structural design and applications. Finally, we discuss the potentials and limitations of the traditional and alternative plasmonic materials for applications in enhanced spectroscopy and photocatalysis.

Surface-Enhanced Raman Scattering Optophysiology Nanofibers for the Detection of Heavy Metals in Single Breast Cancer Cells

Mercury(II) ions (Hg²⁺) and silver ions (Ag⁺) are two of the most hazardous pollutants causing serious damage to human health. Here, we constructed surface-enhanced Raman scattering (SERS)-active nanofibers covered with 4-mercaptopyridine (4-Mpy)-modified gold nanoparticles to detect Hg²⁺ and Ag⁺. Experimental evidence suggests that the observed spectral changes originate from the combined effect of (i) the coordination between the nitrogen on 4-Mpy and the metal ions and (ii) the 4-Mpy molecular orientation (from flatter to more perpendicular with respect to the metal surface). The relative intensity of a pair of characteristic Raman peaks (at ∼428 and ∼708 cm^-1) was used to quantify the metal ion concentration, greatly increasing the reproducibility of the measurement compared to signal-on or signal-off detection based on a single SERS peak. The detection limit of this method for Hg²⁺ is lower than that for the Ag⁺ (5 vs 100 nM), which can be explained by the stronger interaction energy between Hg²⁺ and N compared to Ag⁺ and N, as demonstrated by density functional theory calculations. The Hg²⁺ and Ag⁺ ions can be masked by adding ethylenediaminetetraacetate and Cl^-, respectively, to the Hg²⁺ and Ag⁺ samples. The good sensitivity, high reproducibility, and excellent selectivity of these nanosensors were also demonstrated. Furthermore, detection of Hg²⁺ in living breast cancer cells at the subcellular level is possible, thanks to the nanometric size of the herein described SERS nanosensors, allowing high spatial resolution and minimal cell damage.

Competitive immunosensor for sensitive and optical anti-interference detection of imidacloprid by surface-enhanced Raman scattering

The sensitive detection of pesticides in complex environment is important but still challenging in presence of organic-rich water sample and food matrix. Herein, we reported a nitrile-mediated SERS immunosensor for sensitive and optical anti-interference determination of imidacloprid. Raman tag contained CN bond could provide a sharp characteristic peak in the Raman-silent spectral window (1800 ~ 2800 cm^-1), which could resist the optical noises from the fingerprint region (<1800 cm^-1). Au_core-Ag_shell bimetallic nanocuboid (AuNR@Ag) connected with antigen and Raman tag was used as Raman probe, while Fe₃O₄ magnetic nanoparticle functionalized with anti-imidacloprid antibody was applied as signal enhancer. Owing to the specific recognition ability between antigen and antibody, the competitive system with imidacloprid was formed. Under the optimal condition, the linear relationship was developed in the range of 10-400 nM. Finally, the SERS immunosensor was successfully applied to determine imidacloprid in real samples with recoveries from 96.8% to 100.5%.

Polarized SERS Controlled by Anisotropic Growth on Ordered Curvature Substrate

Colloidal lithography is an efficient and low-cost method to prepare an ordered nanostructure array with new shapes and properties. In this study, square-shaped and cone-shaped Au nanostructures were obtained by 70° angle deposition onto polystyrene bead array with the diameter of 500 nm when a space of 120 nm is created between the neighbor beads by plasma etching. The gaps between the units decrease when the Au deposition time increases, which leads to the polarized enhanced local field, in agreement with the surface-enhanced Raman scattering spectra (SERS) observations and finite-difference time-domain (FDTD) simulations. When the Au deposition time increased to 5 min, 5 nm gaps form between the neighbor units, which gave an enhancement factor of 5 × 10⁹. The SERS chip was decorated for the detection of the liver cancer cell marker Alpha-fetoprotein (AFP) with the detection limit down to 5 pg/mL.

Silver-nanoparticle-grafted silicon nanocones for reproducible Raman detection of trace contaminants in complex liquid environments

Super-hydrophobic delivery (SHD) is an efficient approach to enrich trace analytes into hot spot regions for ultrasensitive surface-enhanced Raman scattering (SERS) detection. In this article, we propose an efficient and simple method to prepare a highly-uniform SHD-SERS platform of high performance in trace detection, named as "silver-nanoparticle-grafted silicon nanocones" (termed AgNPs/SiNC) platform. It is fabricated via droplet-confined electroless deposition on the super-hydrophobic SiNC array. The AgNPs/SiNC platform allows trace analytes enriched into hot spots formed by AgNPs, leading to an excellent reproducibility and sensitivity. The relative standard deviation (RSD) for detecting R6G (10^-7 M) is down to 4.70% and the lowest detection concentration for R6G is 10^-14 M. Moreover, various contaminants in complex liquid environments, such as, crystal violet (10^-9 M) in lake water, melamine (10^-7 M) in liquid milk and methyl parathion (10^-7 M) in tap water, can be detected using the SERS platform. This result demonstrates the great potential of the AgNPs/SiNC platform in the fields of food safety and environmental monitoring.

AgNPs and MIL-101(Fe) self-assembled nanometer materials improved the SERS detection sensitivity and reproducibility

Recently, in the research of Surface-enhanced Raman scattering (SERS) technology, it is found that the preparation of enhanced substrate is particularly important. In this work, the most commonly used methods were used to synthesize AgNPs and MIL-101(Fe), and AgNPs/MIL-101(Fe) nanocomposite was obtained through self-assembly of the two substances. Four different probe molecules were detected with the self-assembled substrate and compared with the results of same probe molecules with AgNPs and MIL-101(Fe) as SERS substrate separately, it was found that AgNPs/ MIL-101 (Fe) nanocomposites had a strong enhancing effect as SERS substrate. The Enhancement Factor (EF) value of 10^-6 mol/L Rhodamine 6G (R6G) was calculated as 2.09 × 10⁹, and the Raman intensities of the peak relative standard deviation (RSD) of R6G Raman attribution was calculated as 7.55%. The time stability of the material was studied and it was found that the reduced Raman signal and poor reproducibility were due to the AgNPs placement time. AgNPs/ MIL-101 (Fe) nanocomposites were used as SERS substrate to detect Paraquat with a minimum concentration of 10^-12 mol/L. The signal values of Paraquat Raman detected at 10^-6 mol/L in different pH environments were relatively stable.

Interfacial interactions of SERS-active noble metal nanostructures with functional ligands for diagnostic analysis of protein cancer markers

Noble metal nanostructures with designed hot spots have been widely investigated as surface-enhanced Raman spectroscopy (SERS)-active substrates, particularly for selective and sensitive detection of protein cancer markers. For specific target recognition and efficient signal amplification, SERS probe design requires a choice of SERS-active nanostructures as well as their controlled functionalization with Raman dyes and target recognition entities such as antibodies. However, the chemical conjugation of antibodies and Raman dyes to SERS substrates has rarely been discussed to date, despite their substantial roles in detection schemes. The interfacial interactions of metal nanostructures with functional ligands during conjugation are known to be strongly influenced by the various chemical and physical properties of the ligands, such as size, molecular weight, surface charge, 3-dimensional structures, and hydrophilicity/hydrophobicity. In this review, we discuss recent developments in the design of SERS probes over the last 4 years, focusing on their conjugation chemistry for functionalization. A strong preference for covalent bonding is observed with Raman dyes having simpler molecular structures, whereas more complicated ones are non-covalently adsorbed. Antibodies are both covalently and non-covalently bonded to nanostructures, depending on their activity in the SERS probes. Considering that ligand conjugation is highly important for chemical stability, biocompatibility, and functionality of SERS probes, this review is expected to expand the understanding of their interfacial design, leading to SERS as one of the most promising spectroscopic analytical tools for the early detection of protein cancer markers.

Metal-Modified Montmorillonite as Plasmonic Microstructure for Direct Protein Detection

Thanks to its negative surface charge and high swelling behavior, montmorillonite (MMT) has been widely used to design hybrid materials for applications in metal ion adsorption, drug delivery, or antibacterial substrates. The changes in photophysical and photochemical properties observed when fluorophores interact with MMT make these hybrid materials attractive for designing novel optical sensors. Sensor technology is making huge strides forward, achieving high sensitivity and selectivity, but the fabrication of the sensing platform is often time-consuming and requires expensive chemicals and facilities. Here, we synthesized metal-modified MMT particles suitable for the bio-sensing of self-fluorescent biomolecules. The fluorescent enhancement achieved by combining clay minerals and plasmonic effect was exploited to improve the sensitivity of the fluorescence-based detection mechanism. As proof of concept, we showed that the signal of fluorescein isothiocyanate can be harvested by a factor of 60 using silver-modified MMT, while bovine serum albumin was successfully detected at 1.9 µg/mL. Furthermore, we demonstrated the versatility of the proposed hybrid materials by exploiting their plasmonic properties to develop liquid label-free detection systems. Our results on the signal enhancement achieved using metal-modified MMT will allow the development of highly sensitive, easily fabricated, and cost-efficient fluorescent- and plasmonic-based detection methods for biomolecules.

Unravelling the Structure of a Medium-Sized Metalloid Gold Nanocluster and its Filming Property

Unravelling the structure of thiolated metalloid gold nanoclusters in the medium-sized range by single crystal X-ray crystallography (SCXC) is challenging. Herein, we successfully synthesized a novel Au₆₇ (SR)₃₅ nanocluster, and unravelled its single crystal structure by SCXC, which features a mix-structured Au₄₈ kernel protected by one Au₄ (SR)₅ staple and fifteen Au(SR)₂ staples. Unprecedentedly, this structure can be thermally induced to aggregate into larger nanoparticles and self-deposit to form a gold nanoparticles film onto the walls of a vial or other substrates such as quartz, mica or ceramic, which can be developed into a facile, substrate-universal and scalable filming method. The film exhibits high sensitivity, uniformity and recyclability as a surface-enhanced Raman scattering (SERS) substrate and can be applied for detecting multiple organic pollutants.

Recent developments on gold nanostructures for surface enhanced Raman spectroscopy: Particle shape, substrates and analytical applications. A review

Surface enhanced Raman spectroscopy (SERS) is a powerful technique for sensitive analysis which is attracting great attention in the last decades. In this review, different gold nanostructures that have been exploited for SERS analysis are described, ranging from gold nanospheres to anisotropic and complex-shaped gold nanostructures, in which the presence of high aspect ratio features leads to an increment of the electromagnetic field at the surface of the nanomaterial, resulting in enhanced SERS response. In addition to the shape of the nanostructure, the interparticle nanogaps play a prominent role in the SERS efficiency. In this sense, different approaches such as nanoaggregation and formation of assemblies and ordered structures lead to the creation of the so-called hot spots. SERS measurements may be performed in solution, while usually the nanostructures are deposited building a SERS substrate, which can be created via attachment of chemically prepared gold nanostructures, as well as via top-down physical methods. Among the classical supports for creating the SERS substrates, in the last years there is a trend towards the development of flexible supports based on polymers as well as paper. Finally, some recent applications of gold nanostructures-based SERS substrates within the analytical field are discussed to spotlight the potential of this technique in real-world analytical scenarios.