MOF-derived AuNS/LDH with high adsorption ability for surface enhanced Raman spectroscopy detection

Using a simple two step fabrication process substrates with a large and uniform Raman enhancement, based on flexible free standing nanopillars can be manufactured over large areas using readily available silicon processing equipment.

In principle, surface enhanced Raman spectroscopy (SERS) is thought to provide unique identification of a target analyte, even in complex samples or in the presence of multiple analytes. In practice, however, this is not always true for real-world samples due to various forms of interference. In this report, we build upon our previous work on inkjet-printed SERS substrates by using paper and polymer membranes to integrate sample cleanup and analyte separation with SERS detection. Inkjet-printed paper SERS substrates provide a highly sensitive chemical detection platform of unprecedented cost and simplicity. In addition, paper inherently provides unique capabilities, such as capillary-actuated fluid transport and selective molecular retention. Utilizing these properties, we demonstrate two-dimensional chromatographic separation and SERS detection on inkjet-printed paper SERS substrates. Then, we leverage the separation properties of paper and polymer membranes for real applications that feature complex sample matrices, including the detection of down to 5 ppm melamine in infant formula, as well as the quantification of nanograms of heroin in samples contaminated with a highly fluorescent background. The results presented here demonstrate that inkjet-printed paper SERS devices not only provide advantages in terms of sensitivity and cost, but the paper provides inherently integrated sample cleanup capabilities that are not available in traditional SERS substrates and microfluidic SERS devices. These unique capabilities of paper SERS devices enable the identification of targeted analytes even in complex real-world samples.

Surface enhanced Raman spectroscopy (SERS) provides a useful sensory platform whereby target molecules at low concentration are identified, potentially detecting a single molecule. As a result, SERS has been widely applied in a variety of research endeavors using different substrates, one of which is a cellulose-based substrate. The unique properties of cellulose in its various forms make it an important component in the design of a SERS substrate. Being a flexible substrate with minimal SERS signal interference, paper-based cellulose templates are the most extensively explored form of cellulose in SERS substrate design, with innovative designs and applications. This review provides an overview of the fundamental tools of SERS enhancement, followed by a comprehensive appraisal of the various design principles associated with producing cellulose-based materials and their use as SERS substrates. Though cellulose in its various forms cannot provide the localized surface plasmon resonance required in SERS, it aids in aggregation and stabilization of plasmonic nanoparticles leading to “hot spots” for SERS signal enhancement. The unconventional techniques adopted in the designs are examined and the associated challenges are highlighted. The review demonstrates SERS applications of the substrates in diverse technologies such as bioanalysis, water quality assessment, food safety, adulteration of illicit drugs and dye identification in artworks. Ultimately, we envisage the need for a universal set standard to realize the ideal of designing SERS substrates from the perspective of end-user demand. This can be achieved through a re-evaluation of existing findings on cellulose-based SERS substrates.

Black silicon SERS substrate: Effect of surface morphology on SERS detection and application of single algal cell analysis

Tuning the aggregation of silver nanoparticles with carbon dots for the surface-enhanced Raman scattering application

Organochlorine pesticides (OCPs) embody highly lipophilic hazardous chemicals that are being phased out globally. Due to their persistent nature, they are still contaminating the environment, being classified as persistent organic pollutants (POPs). They bioaccumulate through bioconcentration and biomagnification, leading to elevated concentrations at higher trophic levels. Studies show that human long-term exposure to OCPs is correlated with a large panel of common chronic diseases. Due to toxicity concerns, most OCPs are listed as persistent organic pollutants (POPs). Conventionally, separation techniques such as gas chromatography are used to analyze OCPs (e.g., gas chromatography coupled with mass spectrometry (GC/MS)) or electron capture detection (GC/ECD). These are accurate, but expensive and time-consuming methods, which can only be performed in centralized lab environments after extensive pretreatment of the collected samples. Thus, researchers are continuously fueling the need to pursue new faster and less expensive alternatives for their detection and quantification that can be used in the field, possibly in miniaturized lab-on-a-chip systems. In this context, surface enhanced Raman spectroscopy (SERS) represents an exceptional analytical tool for the trace detection of pollutants, offering molecular fingerprint-type data and high sensitivity. For maximum signal amplification, two conditions are imposed: an efficient substrate and a high affinity toward the analyte. Unfortunately, due to the highly hydrophobic nature of these pollutants (OCPs,) they usually have a low affinity toward SERS substrates, increasing the challenge in their SERS detection. In order to overcome this limitation and take advantage of on-site Raman analysis of pollutants, researchers are devising ingenious strategies that are synthetically discussed in this review paper. Aiming to maximize the weak Raman signal of organochlorine pesticides, current practices of increasing the substrate’s performance, along with efforts in improving the selectivity by SERS substrate functionalization meant to adsorb the OCPs in close proximity (via covalent, electrostatic or hydrophobic bonds), are both discussed. Moreover, the prospects of multiplex analysis are also approached. Finally, other perspectives for capturing such hydrophobic molecules (MIPs—molecularly imprinted polymers, immunoassays) and SERS coupled techniques (microfluidics—SERS, electrochemistry—SERS) to overcome some of the restraints are presented.

Plasmonic Au-NPs photodecorated on NiCoLDH nanosheets as a flexible SERS sensor for the real-time detection of fipronil

Surface enhanced Raman spectroscopy (SERS) allows detection of unique signatures of molecular vibrations, which identify molecules adsorbed on nanostructured substrates. However, for SERS detection techniques to be reproducible, highly regular metallic nanostructures must be fabricated. In this work, the authors have optimized high resolution electron beam lithography (EBL) to fabricate gold nanostructures on fused silica substrates for efficient SERS biodetection. 30 keV EBL exposures employing an appropriate anticharging scheme followed by development have been used to fabricate arrays of nanopits in PMMA on a fused silica substrate, with subsequent evaporation of Au and lift-off to obtain arrays of Au dots. The EBL process has been tuned to obtain arrays of dots with a 50 nm pitch and different interdot gaps. The SERS substrates have been biofunctionalized by self-assembled monolayers (of 11-mercaptoundecanoic acid) and a biological analyte protein A immobilized on the substrates. In order to detect the protein in its natural environment, the biofunctionalized samples were maintained in deionized water. SERS spectra of protein A were subsequently acquired allowing detection of many vibrational modes of protein A in aqueous solution.

Advancements in reusable SERS substrates for trace analysis applications

Gold nanostars(Au NSs) are asymmetric anisotropic nanomaterials with sharp edge structure. As a promising branched nanomaterial, Au NS has excellent plasmonic absorption and scattering properties. In order to tune the plasmonic photothermal and surface-enhanced Raman scattering(SERS) activity of Au NSs to obtain the desired characteristics, the effects of reagents on the local surface plasmon resonance(LSPR) bands of Au NSs were studied and the morphology and size were regulated. Nanoparticles with different sharp edges were synthesized to make their local plasmon resonance mode tunable in the visible and near-infrared region. The effects of the number and sharpness of different tips under the control of AgNO3 on the photothermal response of Au NSs and the SERS activity and their mechanism were discussed in detail. The results show that as the length of the branch tip becomes longer and the sharpness increases, the plasmonic photothermal effect of Au NSs is strengthened, and the photothermal conversion efficiency is the highest up to 40% when the length of Au NSs is the longest. Au NSs with high SERS activity are used for the Raman detection substrate. Based on this property, the quantitative detection of the pesticide thiram is achieved.

Stealth modified bottom up SERS substrates for label-free therapeutic drug monitoring of doxorubicin in blood serum

This study achieved single molecule detection, for the first time, on the prefabricated substrate for SERS (surface enhanced Raman spectroscopy) with non-resonant molecules. The substrate with directionally arrayed gold nanoparticle dimers enables huge Raman enhancement and is expected for single molecule identification and structural analysis of a wide range of bio/chemical molecules. Due to remarkable capabilities of rapid and highly-sensitive structural analysis and identification of bio/chemical molecules, SERS is expected as a powerful tool for single molecule analysis. Conventionally molecularly bridged metal nanoparticle aggregates has been used for single molecule SERS so far. In these demonstrations, the aggregates are generated mixing nanoparticle colloid and target analyte solutions with hour-level long incubation time. Rhodamine 6G or crystal violet, which offers a resonant Raman effect, has been used. Therefore, for example, this technique cannot be applied to in-situ real-time analysis of various bio/chemical molecules such as hazardous substances, cell products and nucleobases in microfluidic devices. Although prefabricated SERS structures such as particle aggregates on a substrate are appropriate for the above application, no demonstration of single molecule SERS has been reported. It is because the reported SERS substrate has an insufficient Raman enhancementfactor for single molecule SERS. In this study, for the first time, we demonstrate single molecule SERS of non-resonant molecules using the prefabricated substrate. In order to improve Raman enhancement factor of SERS substrates, this study uses the nano-engineered SERS substrate on which particle dimers are directionally arrayed to match all dimers axes to a polarization direction of incident light. This arrangement is expected to enable single-molecule SERS analysis, because of the large Raman enhancement achieved by coupling with the polarization direction. Arraying the dimers directionally, the huge enhancement can be achieved at all dimers. The substrate was fabricated using nanotrench-guided self-assembly. At the beginning of this fabrication, a colloidal particle solution was injected between a cover glass and a template substrate of Si with nanotrench template. On drying the aqueous particle dispersion between the substrates, the water surface line moved backward and the particles became concentrated near the edge of the meniscus. The drag force pressed the particles onto the template substrate. When the meniscus passed over the templates, particles were trapped on the template. Gold nanoparticles with mean diameters of 100 nm were used in this study. 4,4'-Bipyridine molecule, which is non-Raman resonance molecule and a pesticide material, was used for demonstrating single molecule detection. We performed around 50 Raman measurements for each measurement time (1 s or 0.05 s) and molecule concentration (10−5 or 10−11 M). 10−11 M corresponds to one molecule per volume of a cube with a side 5.5 µm. The statistical analysis was performed. All spectra were discriminated based on with or without a peak at around 1609 cm−1, which is one of the Raman shifts derived from the target molecule. At 10−5 M concentration, all spectra exhibited clear peaks. The distribution of Raman intensities were fitted by one Gaussian curve. At 10−11 M concentration, the experimental data at 1 s and 0.05 s were fitted by three and two Gaussian curves, respectively. These are consistent with a Poisson distribution. The change in the statistical distribution from Gaussian (10−5 M) to Poisson (10−11 M) with decreasing the concentration reflects the probability of detecting 0 (background noise), 1, 2 molecule(s) at 1 s. Two Gaussian curves indicate 0 and 1 molecule at 0.05 s. Average relative intensities of 0, 1 and 2 molecule(s) were 0.89, 1.06, and 1.28 at 1 s, respectively. The net relative intensities of 2 molecules are calculated to 1.28−0.89=0.39, which is 1.8 times as much as 1 molecule, 1.28−1.06=0.22. From these experimental data and statistical analysis, we confirmed that the developed substrates achieved single molecule SERS detection. Finally, nucleobases identification with single molecule sensitivity was demonstrated using 10−11 M solutions of adenine and cytosine. The obtained peaks were enough clear to identify the nucleobases. Therefore, a wide range of bio/chemical molecules is expected to be structurally analyzed and identified by single molecule SERS on the prefabricated substrate.

Ultrasensitive surface-enhanced Raman spectroscopy detection of gaseous sulfur-mustard simulant based on thin oxide-coated gold nanocone arrays

Highly sensitive and flexible inkjet printed SERS sensors on paper

Enhancement of SERS effect in Graphene-Silver hybrids