Antitags: nanostructured tools for developing SERS-based ELISA analogs

Precision engineering of nano-assemblies in superfluid helium by the use of van der Waals forces

The ability to precisely engineer nanostructures underpins a wide range of applications in areas such as electronics, optics, and biomedical sciences. Here we present a novel approach for the growth of nanoparticle assemblies that leverages the unique properties of superfluid helium. Unlike viscous solvents at or near room temperature, superfluid helium provides an unperturbed and cold environment in which weak van der Waals interactions between molecular templates and metal atoms become significant and can define the spatial arrangement of nanoparticles. To demonstrate this concept, diol and porphyrin-based molecules are employed as templates to grow gold nanoparticle assemblies in superfluid helium droplets. After soft-landing on a solid surface to remove the helium, transmission electron microscopy (TEM) imaging shows the growth of gold nanoparticles at specific binding sites within the molecular templates where the interaction between gold atoms and the molecular template is at its strongest.

Combining Acoustic Bioprinting with AI-Assisted Raman Spectroscopy for High-Throughput Identification of Bacteria in Blood

Identifying pathogens in complex samples such as blood, urine, and wastewater is critical to detect infection and inform optimal treatment. Surface-enhanced Raman spectroscopy (SERS) and machine learning (ML) can distinguish among multiple pathogen species, but processing complex fluid samples to sensitively and specifically detect pathogens remains an outstanding challenge. Here, we develop an acoustic bioprinter to digitize samples into millions of droplets, each containing just a few cells, which are identified with SERS and ML. We demonstrate rapid printing of 2 pL droplets from solutions containing S. epidermidis, E. coli, and blood; when they are mixed with gold nanorods (GNRs), SERS enhancements of up to 1500× are achieved.We then train a ML model and achieve ≥99% classification accuracy from cellularly pure samples and ≥87% accuracy from cellularly mixed samples. We also obtain ≥90% accuracy from droplets with pathogen:blood cell ratios <1. Our combined bioprinting and SERS platform could accelerate rapid, sensitive pathogen detection in clinical, environmental, and industrial settings.

Paper-Based Substrate for a Surface-Enhanced Raman Spectroscopy Biosensing Platform-A Silver/Chitosan Nanocomposite Approach

Paper is a popular platform material in all areas of sensor research due to its porosity, large surface area, and biodegradability, to name but a few. Many paper-based nanocomposites have been reported in the last decade as novel substrates for surface-enhanced Raman spectroscopy (SERS). However, there are still limiting factors, like the low density of hot spots or loss of wettability. Herein, we designed a process to fabricate a silver-chitosan nanocomposite layer on paper celluloses by a layer-by-layer method and pH-triggered chitosan assembly. Under microscopic observation, the resulting material showed a nanoporous structure, and silver nanoparticles were anchored evenly over the nanocomposite layer. In SERS measurement, the detection limit of 4-aminothiophenol was 5.13 ppb. Furthermore, its mechanical property and a strategy toward further biosensing approaches were investigated.

Recent progress in mycotoxins detection based on surface-enhanced Raman spectroscopy

Mycotoxins are toxic compounds naturally produced by certain types of fungi. The contamination of mycotoxins can occur on numerous foodstuffs, including cereals, nuts, fruits, and spices, and pose a major threat to humans and animals by causing acute and chronic toxic effects. In this regard, reliable techniques for accurate and sensitive detection of mycotoxins in agricultural products and food samples are urgently needed. As an advanced analytical tool, surface-enhanced Raman spectroscopy (SERS), presents several major advantages, such as ultrahigh sensitivity, rapid detection, fingerprint-type information, and miniaturized equipment. Benefiting from these merits, rapid growth has been observed under the topic of SERS-based mycotoxin detection. This review provides a comprehensive overview of the recent achievements in this area. The progress of SERS-based label-free detection, aptasensor, and immunosensor, as well as SERS combined with other techniques, has been summarized, and in-depth discussion of the remaining challenges has been provided, in order to inspire future development of translating the techniques invented in scientific laboratories into easy-to-operate analytic platforms for rapid detection of mycotoxins.

Machine learning algorithms enhance the specificity of cancer biomarker detection using SERS-based immunoassays in microfluidic chips

Specificity is a challenge in liquid biopsy and early diagnosis of various diseases. There are only a few biomarkers that have been approved for use in cancer diagnostics; however, these biomarkers suffer from a lack of high specificity. Moreover, determining the exact type of disorder for patients with positive liquid biopsy tests is difficult, especially when the aberrant expression of one single biomarker can be found in various other disorders. In this study, a SERS-based protein biomarker detection platform in a microfluidic chip and two machine learning algorithms (K-nearest neighbor and classification tree) are used to improve the reproducibility and specificity of the SERS-based liquid biopsy assay. Applying machine learning algorithms to the analysis of the expression level data of 5 protein biomarkers (CA19-9, HE4, MUC4, MMP7, and mesothelin) in pancreatic cancer patients, ovarian cancer patients, pancreatitis patients, and healthy individuals improves the chance of recognition for one specific disorder among the aforementioned diseases with overlapping protein biomarker changes. Our results demonstrate a convenient but highly specific approach for cancer diagnostics using serum samples.

Plasmonic molecular assays: Recent advances and applications for mobile health

Plasmonics-based biosensing assays have been extensively employed for biomedical applications. Significant advancements in use of plasmonic assays for the construction of point-of-care (POC) diagnostic methods have been made to provide effective and urgent health care of patients, especially in resourcelimited settings. This rapidly progressive research area, centered on the unique surface plasmon resonance (SPR) properties of metallic nanostructures with exceptional absorption and scattering abilities, has greatly facilitated the development of cost-effective, sensitive, and rapid strategies for disease diagnostics and improving patient healthcare in both developed and developing worlds. This review highlights the recent advances and applications of plasmonic technologies for highly sensitive protein and nucleic acid biomarker detection. In particular, we focus on the implementation and penetration of various plasmonic technologies in conventional molecular diagnostic assays, and discuss how such modification has resulted in simpler, faster, and more sensitive alternatives that are suited for point-of-use. Finally, integration of plasmonic molecular assays with various portable POC platforms for mobile health applications are highlighted.

Development of a SERS strategy to overcome the nanoparticle stabilisation effect in serum-containing samples: Application to the quantification of dopamine in the culture medium of PC-12 cells

The analysis of serum samples by surface-enhanced Raman spectroscopy (SERS) has gained ground over the last few years. However, the stabilisation of colloids by the proteins contained in these samples has restricted their use in common practice, unless antibodies or aptamers are used. Therefore, this work was dedicated to the development of a SERS methodology allowing the analysis of serum samples in a simple and easy-to-implement way. This approach was based on the pre-aggregation of the colloid with a salt solution. Gold nanoparticles (AuNPs) were used as the SERS substrate and, owing to its physiopathological importance, dopamine was chosen as a model to implement the SERS approach. The presence of this neurotransmitter could be determined in the concentration range 0.5-50 ppm (2.64-264 µM) in the culture medium of PC-12 cells, with a R² of 0.9874, and at even lower concentrations (0.25 ppm, 1.32 µM) in another matrix containing fewer proteins. Moreover, the effect of calcium and potassium on the dopamine exocytosis from PC-12 cells was studied. Calcium was shown to have a predominant and dose-dependant effect. Finally, PC-12 cells were exposed to dexamethasone in order to increase their biosynthesis and release of dopamine. This increase was monitored with the developed SERS approach.

Nanoplasmonic sensors for detecting circulating cancer biomarkers

The detection of cancer biomarkers represents an important aspect of cancer diagnosis and prognosis. Recently, the concept of liquid biopsy has been introduced whereby diagnosis and prognosis are performed by means of analyzing biological fluids obtained from patients to detect and quantify circulating cancer biomarkers. Unlike conventional biopsy whereby primary tumor cells are analyzed, liquid biopsy enables the detection of a wide variety of circulating cancer biomarkers, including microRNA (miRNA), circulating tumor DNA (ctDNA), proteins, exosomes and circulating tumor cells (CTCs). Among the various techniques that have been developed to detect circulating cancer biomarkers, nanoplasmonic sensors represent a promising measurement approach due to high sensitivity and specificity as well as ease of instrumentation and operation. In this review, we discuss the relevance and applicability of three different categories of nanoplasmonic sensing techniques, namely surface plasmon resonance (SPR), localized surface plasmon resonance (LSPR) and surface-enhanced Raman scattering (SERS), for the detection of different classes of circulating cancer biomarkers.

Simultaneous Surface-Enhanced Raman Spectroscopy Detection of Multiplexed MicroRNA Biomarkers

Simultaneous detection of cancer biomarkers holds great promise for the early diagnosis of cancer. In the present work, an ultrasensitive and reliable surface-enhanced Raman scattering (SERS) sensor has been developed for simultaneous detection of multiple liver cancer related microRNA (miRNA) biomarkers. We first proposed a novel strategy for the synthesis of nanogap-based SERS nanotags by modifying gold nanoparticles (AuNPs) with thiolated DNA and nonfluorescent small encoding molecules. We also explored a simple approach to a green synthesis of hollow silver microspheres (Ag-HMSs) with bacteria as templates. On the basis of the sandwich hybridization assay, probe DNA-conjugated SERS nanotags used as SERS nanoprobes and capture DNA-conjugated Ag-HMSs used as capture substrates were developed for the detection of target miRNA with a detection limit of 10 fM. Multiplexing capability for simultaneous detection of the three liver cancer related miRNAs with the high sensitivity and specificity was demonstrated using the proposed SERS sensor. Furthermore, the practicability of the SERS sensor was supported by the successful determination of target miRNA in cancer cells. The experimental results indicated that the proposed strategy holds significant potential for multiplex detection of cancer biomarkers and offers the opportunity for future applications in clinical diagnosis.

Ultrasensitive fluorescence immunoassay for detection of ochratoxin A using catalase-mediated fluorescence quenching of CdTe QDs

Herein, for the first time we report an improved competitive fluorescent enzyme linked immunosorbent assay (ELISA) for the ultrasensitive detection of ochratoxin A (OTA) by using hydrogen peroxide (H2O2)-induced fluorescence quenching of mercaptopropionic acid-modified CdTe quantum dots (QDs). In this immunoassay, catalase (CAT) was labeled with OTA as a competitive antigen to connect the fluorescence signals of the QDs with the concentration of the target. Through the combinatorial use of H2O2-induced fluorescence quenching of CdTe QDs as a fluorescence signal output and the ultrahigh catalytic activity of CAT to H2O2, our proposed method could be used to perform a dynamic linear detection of OTA ranging from 0.05 pg mL(-1) to 10 pg mL(-1). The half maximal inhibitory concentration was 0.53 pg mL(-1) and the limit of detection was 0.05 pg mL(-1). These values were approximately 283- and 300-folds lower than those of horseradish peroxidase (HRP)-based conventional ELISA, respectively. The reported method is accurate, highly reproducible, and specific against other mycotoxins in agricultural products as well. In summary, the developed fluorescence immunoassay based on H2O2-induced fluorescence quenching of CdTe QDs can be used for the rapid and highly sensitive detection of mycotoxins or haptens in food safety monitoring.

Enhanced ELISA using a handheld pH meter and enzyme-coated microparticles for the portable, sensitive detection of proteins

This work describes a general methodology for enhanced enzyme-linked immunosorbent assay (ELISA) that integrates enzyme-coated microparticle probes for robust yet highly efficient signal amplification and a handheld pH meter for a simple, portable, and quantitative readout.

Plasmonic-ELISA: expanding horizons

Convergence of localized surface plasmon resonance of metal nanoparticles with classical ELISA has emerged as a new class of immunoassays, i.e. plasmonic ELISA, enabling biocatalysis mediated ultrasensitive naked-eye detection of disease biomarkers.

Preparation of a Superhydrophobic and Peroxidase-like Activity Array Chip for H2O2 Sensing by Surface-Enhanced Raman Scattering

In this paper, we propose a novel and simple method for preparing a dual-biomimetic functional array possessing both superhydrophobic and peroxidase-like activity that can be used for hydrogen peroxide (H2O2) sensing. The proposed method is an integration innovation that combines the above two properties and surface-enhanced Raman scattering (SERS). We integrated a series of well-ordered arrays of Au points (d = 1 mm) onto a superhydrophobic copper (Cu)/silver (Ag) surface by replicating an arrayed molybdenum template. Instead of using photoresists and the traditional lithography method, we utilized a chemical etching method (a substitution reaction between Cu and HAuCl4) with a Cu/Ag superhydrophobic surface as the barrier layer, which has the benefit of water repellency. The as-prepared Au points were observed to possess peroxidase-like activity, allowing for catalytic oxidation of the chromogenic molecule o-phenylenediamine dihydrochloride (OPD). Oxidation was evidenced by a color change in the presence of H2O2, which allows the array chip to act as an H2O2 sensor. In this study, the water repellency of the superhydrophobic surface was used to fabricate the array chip and increase the local reactant concentration during the catalytic reaction. As a result, the catalytic reaction occurred when only 2 μL of an aqueous sample (OPD/H2O2) was placed onto the Au point, and the enzymatic product, 2,3-diaminophenazine, showed a SERS signal distinguishable from that of OPD after mixing with 2 μL of colloidal Au. Using the dual-biomimetic functional array chip, quantitative analysis of H2O2 was performed by observing the change in the SERS spectra, which showed a concentration-dependent behavior for H2O2. This method allows for the detection of H2O2 at concentrations as low as 3 pmol per 2 μL of sample, which is a considerable advantage in H2O2 analysis. The as-prepared substrate was convenient for H2O2 detection because only a small amount of sample was required in each analysis. Highly sensitive detection was realized using SERS. Therefore, this chip was shown to exhibit significant potential for applications in bioanalysis.

SERS encoded silver pyramids for attomolar detection of multiplexed disease biomarkers

Three disease biomarkers can simultaneously be detected at the attomolar level because of a novel surface-enhanced Raman scattering (SERS) encoded silver pyramid sensing system. This newly designed pyramidal sensor with well-controlled geometry exhibits highly sensitive, selective, and reproducible SERS signals, and holds promising potential for biodetection applications.

SERS-based quantitative detection of ovarian cancer prognostic factor haptoglobin

Surface-enhanced Raman spectroscopy (SERS) is increasingly being used for biosensing because of its high sensitivity and low detection limit, which are made possible by the unique Raman 'fingerprint' spectra from the biomolecules. Here we propose a novel SERS method for the fast, sensitive, and reliable quantitative analysis of haptoglobin (Hp), an acute phase plasma glycoprotein that is widely gaining application as a prognostic ovarian cancer biomarker. We exploited the peroxidase activity of the hemoglobin-haptoglobin (Hb-Hp) complex formed by the selective and specific binding of Hp to free Hb to catalyze the reaction of 3,3',5,5'-tetramethylbenzidine (TMB) substrate and hydrogen peroxide to result in the final product of strongly SERS-active TMB(2+). We observed a linear increase in the SERS signal of TMB(2+) with increasing concentrations of Hb-Hp complex from 50 nM to 34 μM. Based on this concentration-dependent SERS spectrum, we quantified Hp in clinical samples. We observed that our inference about the prognosis of the disease coincided with the histology data and that our method was much more sensitive than the enzyme-linked immunosorbent assay method.

Ag shell-Au satellite hetero-nanostructure for ultra-sensitive, reproducible, and homogeneous NIR SERS activity

It is critical to create isotropic hot spots in developing a reproducible, homogeneous, and ultrasensitive SERS probe. Here, an Ag shell-Au satellite (Ag-Au SS) nanostructure composed of an Ag shell and surrounding Au nanoparticles was developed as a near-IR active SERS probe. The heterometallic shell-satellite structure based SERS probe produced intense and uniform SERS signals (SERS enhancement factor ∼1.4 × 10(6) with 11% relative standard deviation) with high detectability (100% under current measurement condition) by 785 nm photoexcitation. This signal enhancement was independent of the laser polarizations, which reflects the isotropic feature of the SERS activity of Ag-Au SS from the three-dimensional (3D) distribution of SERS hot spots between the shell and the surrounding satellite particles. The Ag-Au SS nanostructure shows a great potential as a reproducible and quantifiable NIR SERS probe for in vivo targets.

Magneto-plasmonic Au-Fe alloy nanoparticles designed for multimodal SERS-MRI-CT imaging

Diagnostic approaches based on multimodal imaging are needed for accurate selection of the therapeutic regimens in several diseases, although the dose of administered contrast drugs must be reduced to minimize side effects. Therefore, large efforts are deployed in the development of multimodal contrast agents (MCAs) that permit the complementary visualization of the same diseased area with different sensitivity and different spatial resolution by applying multiple diagnostic techniques. Ideally, MCAs should also allow imaging of diseased tissues with high spatial resolution during surgical interventions. Here a new system based on multifunctional Au-Fe alloy nanoparticles designed to satisfy the main requirements of an ideal MCA is reported and their biocompatibility and imaging capability are described. The MCAs show easy and versatile surface conjugation with thiolated molecules, magnetic resonance imaging (MRI) and computed X-ray tomography (CT) signals for anatomical and physiological information (i.e., diagnostic and prognostic imaging), large Raman signals amplified by surface enhanced Raman scattering (SERS) for high sensitivity and high resolution intrasurgical imaging, biocompatibility, exploitability for in vivo use and capability of selective accumulation in tumors by enhanced permeability and retention effect. Taken together, these results show that Au-Fe nanoalloys are excellent candidates as multimodal MRI-CT-SERS imaging agents.

Antitags: SERS-encoded nanoparticle assemblies that enable single-spot multiplex protein detection

Simultaneous detection of multiple proteins on a single spot can be efficiently achieved by using multiplexed surface-enhanced Raman spectroscopy (SERS)-encoded nanoparticle 'antitags' consisting of poly(ethylene glycol) (PEG)-protected silver dimers (and higher aggregates) and antibody-tagging entities. The effective SERS-based multivariate deconvolution approach guarantees an accurate and successful distinguishable identification of single and multiple proteins in complex samples. Their potential application in multiplexed SERS bioimaging technology can be easily envisaged.

Using a silver-enhanced microarray sandwich structure to improve SERS sensitivity for protein detection

A simple and sensitive method, based on surface-enhanced Raman scattering (SERS), for immunoassay and label-free protein detection is reported. A series of bowl-shaped silver cavity arrays were fabricated by electrodeposition using a self-assembled polystyrene spheres template. The reflection spectra of these cavity arrays were recorded as a function of film thickness, and then correlated with SERS enhancement using sodium thiophenolate as the probe molecule. The results reveal that SERS enhancement can be maximized when the frequency of both the incident laser and the Raman scattering approach the frequency of the localized surface plasmon resonance. The optimized array was then used as the bottom layer of a silver nanoparticle-protein-bowl-shaped silver cavity array sandwich. The second layer of silver was introduced by the interactions between the proteins in the middle layer of the sandwich architecture and silver nanoparticles. Human IgG bound to the surface of this microcavity array can retain its recognition function. With the Raman reporter molecules labeled on the antibody, a detection limit down to 0.1 ng mL(-1) for human IgG is easily achieved. Furthermore, the SERS spectra of label-free proteins (catalase, cytochrome C, avidin and lysozyme) from the assembled sandwich have excellent reproducibility and high quality. The results reveal that the proposed approach has potential for use in qualitative and quantitative detection of biomolecules.

Surface-enhanced Raman spectroscopy (SERS): progress and trends

Surface-enhanced Raman spectroscopy (SERS) combines molecular fingerprint specificity with potential single-molecule sensitivity. Therefore, the SERS technique is an attractive tool for sensing molecules in trace amounts within the field of chemical and biochemical analytics. Since SERS is an ongoing topic, which can be illustrated by the increased annual number of publications within the last few years, this review reflects the progress and trends in SERS research in approximately the last three years. The main reason why the SERS technique has not been established as a routine analytic technique, despite its high specificity and sensitivity, is due to the low reproducibility of the SERS signal. Thus, this review is dominated by the discussion of the various concepts for generating powerful, reproducible, SERS-active surfaces. Furthermore, the limit of sensitivity in SERS is introduced in the context of single-molecule spectroscopy and the calculation of the 'real' enhancement factor. In order to shed more light onto the underlying molecular processes of SERS, the theoretical description of SERS spectra is also a growing research field and will be summarized here. In addition, the recording of SERS spectra is affected by a number of parameters, such as laser power, integration time, and analyte concentration. To benefit from synergies, SERS is combined with other methods, such as scanning probe microscopy and microfluidics, which illustrates the broad applications of this powerful technique.

Raman response of dithiolated nanoparticle linkers

We report on a study of the overall surface-enhanced Raman scattering (SERS) response from several thiol-terminated molecules located at interstitial sites between Ag nanoparticles. Multiplexing of the SERS signal was demonstrated along with its dependence of the molecular length, regiochemistry, and the number of thiol groups. The data collected establish pathways for the rational design of the SERS reporters and multiplexed sensory applications.