SERS-Active-Charged Microgels for Size- and Charge-Selective Molecular Analysis of Complex Biological Samples

Integrating machine learning and biosensors in microfluidic devices: A review

Microfluidic devices are increasingly widespread in the literature, being applied to numerous exciting applications, from chemical research to Point-of-Care devices, passing through drug development and clinical scenarios. Setting up these microenvironments, however, introduces the necessity of locally controlling the variables involved in the phenomena under investigation. For this reason, the literature has deeply explored the possibility of introducing sensing elements to investigate the physical quantities and the biochemical concentration inside microfluidic devices. Biosensors, particularly, are well known for their high accuracy, selectivity, and responsiveness. However, their signals could be challenging to interpret and must be carefully analysed to carry out the correct information. In addition, proper data analysis has been demonstrated even to increase biosensors' mentioned qualities. To this regard, machine learning algorithms are undoubtedly among the most suitable approaches to undertake this job, automatically learning from data and highlighting biosensor signals' characteristics at best. Interestingly, it was also demonstrated to benefit microfluidic devices themselves, in a new paradigm that the literature is starting to name "intelligent microfluidics", ideally closing this benefic interaction among these disciplines. This review aims to demonstrate the advantages of the triad paradigm microfluidics-biosensors-machine learning, which is still little used but has a great perspective. After briefly describing the single entities, the different sections will demonstrate the benefits of the dual interactions, highlighting the applications where the reviewed triad paradigm was employed.

Coatable strain sensors for nonplanar surfaces

Rapidly fabricating flexible and stretchable sensors on nonplanar surfaces is crucial for wearable device applications. We employed a novel fabrication method, incorporating molds and gels into electroless plating, to enable direct printing of sensors on a wide array of surfaces, from those with up to 100 μm profile heights to hydrogels with a Young's modulus of 100 kPa. This coatable strain (CS) sensor offers several potential advantages. Firstly, it is designed to circumvent the typical limitations of limited flexibility, plastic deformation, and low repeatability found in viscoelastic polymers by being directly coated onto the surface without requiring a substrate. Secondly, it potentially increases the effective contact area and signal-to-noise ratio by eliminating voids between the sensor and the surface. Finally, the CS sensor can obtain any desired patterning at room temperature in a matter of minutes, significantly reducing energy and time consumption. In this study, we demonstrated the versatility of the CS sensor by applying it to a range of substrates, showcasing its adaptability to diverse materials, surface roughness levels, and Young's modulus values. Our primary focus was on plant growth monitoring, a challenging application that showcased the sensor's efficacy on surfaces like needles, hairy leaves, and fruits. These applications, traditionally difficult for conventional polymer-based sensors, serve to illustrate the CS sensor's potential in a range of complex environmental contexts. The successful deployment of the CS sensor in these settings suggests its broader applicability in various scientific and technological fields, potentially contributing to significant developments in the area of wearable devices and beyond.

Micromolding-Assisted Production of SERS-Active Microcylinders for Size- and Charge-Selective Molecular Detection

Surface-enhanced Raman scattering (SERS) is an effective technique for amplifying the Raman signal of molecules by using metal nanostructures. However, these metal surfaces are susceptible to contamination by undesirable adhesives in complex mixtures, typically necessitating a time-consuming and costly sample pretreatment. In order to circumvent this, metal nanoparticles have been uniformly embedded within microgels by using microfluidics. In this work, we introduce a simple, scalable micromolding method for creating SERS-active cylindrical microgels designed to eliminate the need for pretreatment. These microcylinders are created through the simultaneous photoreduction and photo-cross-linking of precursor solutions. These solutions are optimized for consistent, high-intensity Raman signals as well as molecular size and charge selectivity. A sequential micromolding method is employed to design dual-compartment microcylinders, offering additional functionalities such as optical encoding, magnetoresponsiveness, and dual-charge selectivity. These SERS-active microcylinders provide robust Raman signals of small molecules, even in the presence of adhesive proteins, without compromising sensitivity. To demonstrate this capability, we directly detect pyocyanin in saliva and tartrazine in whole milk without any need for sample pretreatment.

"On-site" analysis of pesticide residues in complex sample matrix by plasmonic SERS nanostructure hybridized hydrogel

BACKGROUND

Surface-enhanced Raman spectroscopy (SERS) has been extensively used in biomedical and food safety detection due to its advantages of label-free, in situ and fingerprint spectrum. However, it is challenging to develop an excellent SERS substrate that possesses all three of these characteristics including sensitivity, repeatability and stability.

RESULTS

In this work, a specific sodium alginate hydrogel flexible SERS substrate encapsulated gold-silver core-shell nanoparticles (Au@Ag NPs) was developed to address the aforementioned issue. The Au@Ag NPs with SERS "hot spot" structure were evenly dispersed in the hydrogel, which achieved the direct and high efficiency detection of the pesticide residues from complex sample matrix. Taking thiram as objective, this SERS substrates exhibit high sensitivity (detection limit of approximately 1 × 10-10 mol/L), excellent stability (maintain above 78.35 % of SERS activity after 7 weeks) and outstanding repeatability (RSD in one substrate as low as 3.56 %). Furthermore, the flexible hydrogel SERS substrates can be used to analyze a variety of small molecules in real samples (juices, vegetables and fruits), without the need for a laborious pretreatment process.

SIGNIFICANCE

In light of the aforementioned benefits, the functional flexible hydrogel SERS substrates present a reliable platform for the accurate and on-site measurement of chemical contaminants from complex samples.

Application of SERS in In-Vitro Biomedical Detection

Surface-enhanced Raman scattering (SERS), as a rapid and nondestructive biological detection method, holds great promise for clinical on spot and early diagnosis. In order to address the challenging demands of on spot detection of biomedical samples, a variety of strategies has been developed. These strategies include substrate structural and component engineering, data processing techniques, as well as combination with other analytical methods. This report summarizes the recent SERS developments for biomedical detection, and their promising applications in cancer detection, virus or bacterial infection detection, miscarriage spotting, neurological disease screening et al. The first part discusses the frequently used SERS substrate component and structures, the second part reports on the detection strategies for nucleic acids, proteins, bacteria, and virus, the third part summarizes their promising applications in clinical detection in a variety of illnesses, and the forth part reports on recent development of SERS in combination with other analytical techniques. The special merits, challenges, and perspectives are discussed in both introduction and conclusion sections.

Applications of surface-enhanced Raman spectroscopy in environmental detection

As the human population grows, the anthropogenic impacts from various agricultural and industrial processes produce unwanted contaminants in the environment. The accurate, sensitive and rapid detection of such contaminants is vital for human health and safety. Surface-enhanced Raman spectroscopy (SERS) is a valuable analytical tool with wide applications in environmental contaminant monitoring. The aim of this review is to summarize recent advancements within SERS research as it applies to environmental detection, with a focus on research published or accessible from January 2021 through December 2021 including early-access publications. Our goal is to provide a wide breadth of information that can be used to provide background knowledge of the field, as well as inform and encourage further development of SERS techniques in protecting environmental quality and safety. Specifically, we highlight the characteristics of effective SERS nanosubstrates, and explore methods for the SERS detection of inorganic, organic, and biological contaminants including heavy metals, pharmaceuticals, plastic particles, synthetic dyes, pesticides, viruses, bacteria and mycotoxins. We also discuss the current limitations of SERS technologies in environmental detection and propose several avenues for future investigation. We encourage researchers to fill in the identified gaps so that SERS can be implemented in a real-world environment more effectively and efficiently, ultimately providing reliable and timely data to help and make science-based strategies and policies to protect environmental safety and public health.

Prospects of Surface-Enhanced Raman Spectroscopy for Biomarker Monitoring toward Precision Medicine

Future precision medicine will be undoubtedly sustained by the detection of validated biomarkers that enable a precise classification of patients based on their predicted disease risk, prognosis, and response to a specific treatment. Up to now, genomics, transcriptomics, and immunohistochemistry have been the main clinically amenable tools at hand for identifying key diagnostic, prognostic, and predictive biomarkers. However, other molecular strategies, including metabolomics, are still in their infancy and require the development of new biomarker detection technologies, toward routine implementation into clinical diagnosis. In this context, surface-enhanced Raman scattering (SERS) spectroscopy has been recognized as a promising technology for clinical monitoring thanks to its high sensitivity and label-free operation, which should help accelerate the discovery of biomarkers and their corresponding screening in a simpler, faster, and less-expensive manner. Many studies have demonstrated the excellent performance of SERS in biomedical applications. However, such studies have also revealed several variables that should be considered for accurate SERS monitoring, in particular, when the signal is collected from biological sources (tissues, cells or biofluids). This Perspective is aimed at piecing together the puzzle of SERS in biomarker monitoring, with a view on future challenges and implications. We address the most relevant requirements of plasmonic substrates for biomedical applications, as well as the implementation of tools from artificial intelligence or biotechnology to guide the development of highly versatile sensors.

Microfluidics and surface-enhanced Raman spectroscopy, a win-win combination?

With the continuous development in nanoscience and nanotechnology, analytical techniques like surface-enhanced Raman spectroscopy (SERS) render structural and chemical information of a variety of analyte molecules in ultra-low concentration. Although this technique is making significant progress in various fields, the reproducibility of SERS measurements and sensitivity towards small molecules are still daunting challenges. In this regard, microfluidic surface-enhanced Raman spectroscopy (MF-SERS) is well on its way to join the toolbox of analytical chemists. This review article explains how MF-SERS is becoming a powerful tool in analytical chemistry. We critically present the developments in SERS substrates for microfluidic devices and how these substrates in microfluidic channels can improve the SERS sensitivity, reproducibility, and detection limit. We then introduce the building materials for microfluidic platforms and their types such as droplet, centrifugal, and digital microfluidics. Finally, we enumerate some challenges and future directions in microfluidic SERS. Overall, this article showcases the potential and versatility of microfluidic SERS in overcoming the inherent issues in the SERS technique and also discusses the advantage of adding SERS to the arsenal of microfluidics.

Advances in droplet microfluidics for SERS and Raman analysis

Raman spectroscopy can realize qualitative and quantitative characterization, and surface-enhanced Raman spectroscopy (SERS) can further enhance its detection sensitivity. In combination with droplet microfluidics, some significant but insurmountable limitations of SERS and Raman spectroscopy can be overcome to some extent, thus improving their detection capability and extending their application. During the past decade, these systems have constantly developed and demonstrated a great potential in more applications, but there is no new review systematically summarizing the droplet microfluidics-based Raman and SERS analysis system since the first related review was published in 2011. Thus, there is a great need for a new review to summarize the advances. In this review, we focus on droplet microfluidics-based Raman and SERS analysis, and summarize two mainstream research directions on this topic up to now. The one is SERS or Raman detection in the moving droplet microreactors, including analysis of molecules, single cells and chemical reaction processes. The other one is SERS active microparticle fabrication via microfluidic droplet templates covering polymer matrix and photonic crystal microparticles. We also comment on the advantages, disadvantage and correlation resolution of droplet microfluidics for SERS or Raman. Finally, we summarize these systems and illustrate our perspectives for future research directions in this field.

An Overview on Recent Progress of the Hydrogels: From Material Resources, Properties to Functional Applications

Hydrogels, as the most typical elastomer materials with three-dimensional network structures, have attracted wide attention owing to their outstanding features in fields of sensitive stimulus response, low surface friction coefficient, good flexibility and bio-compatibility. Because of numerous fresh polymer materials (or polymerization monomers), hydrogels with various structure diversities and excellent properties are emerging, and the development of hydrogels is very vigorous over the past decade. This review focuses on state-of-the-art advances, systematically reviews the recent progress on construction of novel hydrogels utilized several kinds of typical polymerization monomers, and explores the main chemical and physical cross-linking methods to develop the diversity of hydrogels. Following the aspects mentioned above, the classification and emerging applications of hydrogels, such as pH response, ionic response, electrical response, thermal response, biomolecular response, and gas response, are extensively summarized. Finally, we have done this review with the promises and challenges for the future evolution of hydrogels and their biological applications. cross-linking methods; functional applications; hydrogels; material resources This article is protected by copyright. All rights reserved.

A capillary-driven LoC-SERS device integrated with catalytic hairpin assembly amplification technology for NSCLC-related biomarkers detection

A capillary-driven LoC-SERS device is fabricated for the ultrasensitive detection of NSCLC-related protein biomarkers with catalytic hairpin assembly as the signal amplification strategy.

Oil-Free Gold Nanobipyramid@Ag Microgels as a Functional SERS Substrate for Direct Detection of Small Molecules in a Complex Sample Matrix

Surface-enhanced Raman scattering (SERS) is a super-sensitive analysis technology based on the target molecular fingerprint information. The enhancement of local electromagnetic field of the SERS substrate would increase the target molecules' Raman intensity which adsorb on the surface of nanoparticles. However, the existing adhesive macromolecules in the complex mixed sample would interfere with the adsorption of small target molecules, and it weakens the Raman intensity of target molecules. Microgels are one of the potential materials to suppress the interference of adhesive macromolecules and to avoid the complex pretreatments. However, most of the current microgel synthesis methods involve complex operations with precise instrumentation or the interference of oil and organic reagents. In this work, a simple and oil-free method was proposed to synthesize the gold nanobipyramid (Au NBP)@Ag@hyaluronic acid microgel via the condensation reaction of carboxyl and amino groups. As a proof-of-concept demonstration for small-molecule detection, the rhodamine 6G (R6G) molecules were allowed to enter inside the microgel through the meshes and adsorb on the surface of Au NBP@Ag nanoparticles within 30 min, while the macromolecule (bovine serum albumin in this case) was retained outside the microgel in the meantime. In addition, under the combined action of lightning rod effect of Au NBP and surface plasmon resonance effect of silver render the microgels with high SERS activity. The synthetic Au NBP@Ag@hyaluronic acid microgels were applied to detect 6-thioguanine in the human serum without any pretreatment process, and it showed a high signal enhancement and stable SERS signal, which can satisfy the requirement of clinical diagnosis. These results show that the proposed microgels have potential applications in the field of point-of-care testing.

Nanocomposite Scaffolds for Monitoring of Drug Diffusion in Three-Dimensional Cell Environments by Surface-Enhanced Raman Spectroscopy

Monitoring dynamic processes in complex cellular environments requires the integration of uniformly distributed detectors within such three-dimensional (3D) networks, to an extent that the sensor could provide real-time information on nearby perturbations in a non-invasive manner. In this context, the development of 3D-printed structures that can function as both sensors and cell culture platforms emerges as a promising strategy, not only for mimicking a specific cell niche but also toward identifying its characteristic physicochemical conditions, such as concentration gradients. We present herein a 3D cancer model that incorporates a hydrogel-based scaffold containing gold nanorods. In addition to sustaining cell growth, the printed nanocomposite inks display the ability to uncover drug diffusion profiles by surface-enhanced Raman scattering, with high spatiotemporal resolution. We additionally demonstrate that the acquired information could pave the way to designing novel strategies for drug discovery in cancer therapy, through correlation of drug diffusion with cell death.

Bio-inspired intestinal scavenger from microfluidic electrospray for detoxifying lipopolysaccharide

Lipopolysaccharide (LPS) plays an important role in metabolic syndrome (MetS) and other gut-derived diseases, and detoxifying LPS is considered to be a fundamental approach to prevent and treat these diseases. Here, inspired by the feeding behaviour of scavenger, novel microfluidic microcapsules with alkaline phosphatase (ALP) encapsulation and the scavenger-like molecular sieve shell are presented for cleaning intestinal LPS. Benefiting from the precisely controlled of the pore size and microfluidic electrospray, the generated microcapsules were imparted with porous molecular-sieve shells and ALP encapsulated active cores. These microcapsules could continuously work as an intestinal scavenger after colonized in intestine. It has been demonstrated that the microcapsules could englobe LPS while inhibit the permeation of digestive enzyme, and this ability contributes to promising ALP's activity, protecting cells at the presence of LPS and reducing inflammation. In addition, this scavenger inspired microcapsule could effectively decrease the LPS in organs, reduce inflammation and regulating fat metabolism in vivo. These features make the ALP encapsulated microcapsules an ideal candidate for treating MetS and other LPS related diseases.

Stimuli-responsive microgels with fluorescent and SERS activities for water and temperature sensing

Design and application of stimulus-responsive microgels is still in its infancy but is an exhilarating topic in controllable sensing device. Here, we have fabricated a dual-responsive platform capable of both sensitive on-spot fluorescence analysis and reliable surface-enhanced Raman scattering (SERS) quantification of water and temperature by in-situ encapsulating 4,4'-dimercaptoazobenzene (DMAB), meso-formyl-1,3,5,7-tetramethyl pyrromethene fluoroborate (FPF) probe and Ag nanoparticles (AgNPs) into polyvinyl alcohol (PVA) microgels. The smart microgels exhibit ultra-sensitive (detection limit 10^-4% v/v) and reversible response towards water due to the liner relationship between network volume and SERS performance of the microgels. Furthermore, the presence of water triggers the conversion of FPF to aldehyde hydrate, facilitating visual assay of trace water in matrix samples through the enhanced fluorescence signals. Interestingly, the SERS signals can be precisely tuned by the thermo-sensitive microgels substrate, thus achieving the temperature monitoring from 32 to 50 °C. The microgels-based sensor has fast-response (2 min), excellent stability, and enables accurate and reliable response of water in organic solvent and pharmaceutical products. As a smart and flexible sensor, the hybrid microgels will facilitate the field of POC analysis, as well as molecular recognition in the future.

Recent progress of microfluidics in surface-enhanced Raman spectroscopic analysis

Surface-enhanced Raman spectroscopy is a significant analytical tool capable of fingerprint identification of molecule in a rapid and ultrasensitive manner. However, it is still hard to meet the requirements of practical sample analysis. The introduction of microfluidics can effectively enhance the performance of surface-enhanced Raman spectroscopy in complex sample analysis including reproducibility, selectivity, sensitivity, and speed. This review summarizes the recent progress of microfluidics in surface-enhanced Raman spectroscopic analysis through four combination approaches. First, microfluidic synthetic techniques offer uniform nano-/microparticle fabrication approaches for reproductive surface-enhanced Raman spectroscopic analysis. Second, the integration of microchip and surface-enhanced Raman spectroscopic substrate provides advanced devices for sensitive and efficient detection. Third, microfluidic sample preparations enable rapid separation and preconcentration of analyte prior to surface-enhanced Raman spectroscopic detection. Fourth, highly integrated microfluidic devices can be employed to realize multistep surface-enhanced Raman spectroscopic analysis containing material fabrication, sample preparation, and detection processes. Furthermore, the challenges and outlooks of the application of microfluidics in surface-enhanced Raman spectroscopic analysis are discussed.

Electrokinetic Preseparation and Molecularly Imprinted Trapping for Highly Selective SERS Detection of Charged Phthalate Plasticizers

Nonspecific binding and weak spectral discernment are the main challenges for surface-enhanced Raman scattering (SERS) detection, especially in real sample analysis. Herein, molecularly imprinted polymer (MIP)-based core-shell AuNP@polydopamine (AuNP@PDA-MIP) nanoparticles (NPs) are designed and immobilized on an electrochemically reduced MoS₂-modified screen-printed electrode (SPE). This portable electrochemical-Raman interface offers the dual functions of electrokinetic preseparation (EP) and MIP trapping of charged molecules so that a reliable SERS recognition with molecular selectivity and high sensitivity can be achieved. Core-shell AuNP@PDA-MIP NPs can be controllably synthesized, possess predesigned specific recognition, and provide "hot spots" at the junction of NPs. The introduction of an electric field enables the autonomous exclusion and separation of similarly charged molecules as well as attraction and concentration of the oppositely charged molecules by electrostatic attraction. Subsequently, the specific MIP recognition cavities allow selective adsorption of targets on the interface without the interference of analogues. Owing to the distinctive design of the multiple coupling separation, trapping, and enrichment strategies, the MIP-based SERS-active interface can be used for label-free detection of charged molecules in real samples without pretreatment. As a proof-of-concept study, label-free SERS detection of charged phthalate plasticizers (PAEs) was demonstrated with a detection limit as low as 2.7 × 10^-12 M for dimethyl phthalate (DMP) and 2.3 × 10^-11 M for di(2-ethylhexyl) phthalate (DEHP). This sensing strategy for in situ SERS analysis of charged pollutants or toxins holds vast promises for a wide range of in-field applications.

Plasmonic Microgels for Raman-Based Molecular Detection Created by Simultaneous Photoreduction and Photocross-linking

Molecular detection in complex mixtures is of great importance in biomedical diagnosis, food safety, and environmental monitoring. Although surface-enhanced Raman scattering serves as one of the most promising detection methods, metal surfaces are prone to contamination, making the direct detection of small molecules in mixtures elusive. Metal nanoparticle-loaded hydrogels have been used for the exclusion of large adhesive molecules and direct detection of small molecules. Here, we design microgels containing highly concentrated gold nanoparticles through the simultaneous formation of hydrogel and gold nanoparticles in emulsion droplets. Monodisperse water-in-oil droplets are microfluidically prepared to contain a gold precursor, hydrogel precursor, and photoinitiator. Upon ultraviolet irradiation, a hydrogel is gradually formed in the drop by photocross-linking at which gold nanoparticles are synthesized and grown by photo and thermal reduction. The in situ synthesis provides the uniform distribution of gold nanoparticles at very high concentrations without aggregation, which is otherwise very difficult to achieve. Using the microgels, small molecules in albumin solutions can be detected by Raman measurement with high signal sensitivity and reproducibility in the absence of interruption from albumin. As a proof of concept, we demonstrate the direct detection of pyocyanin, a biomarker for Pseudomonas infection spiked in unpurified saliva.

Surface-enhanced Raman spectroscopy: benefits, trade-offs and future developments

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique with sensitivity down to the single molecule level that provides fine molecular fingerprints, allowing for direct identification of target analytes. Extensive theoretical and experimental research, together with continuous development of nanotechnology, has significantly broadened the scope of SERS and made it a hot research field in chemistry, physics, materials, biomedicine, and so on. However, SERS has not been developed into a routine analytical technique, and continuous efforts have been made to address the problems preventing its real-world application. The present minireview focuses on analyzing current and potential strategies to tackle problems and realize the SERS performance necessary for translation to practical applications.

Microfluidic Designing Microgels Containing Highly Concentrated Gold Nanoparticles for SERS Analysis of Complex Fluids

Surface-enhanced Raman scattering (SERS) is one of the most promising methods to detect small molecules for point-of-care analysis as it is rapid, nondestructive, label-free, and applicable for aqueous samples. Here, microgels containing highly concentrated yet evenly dispersed gold nanoparticles are designed to provide SERS substrates that simultaneously achieve contamination-free metal surfaces and high signal enhancement and reproducibility. With capillary microfluidic devices, water-in-oil-in-water (W/O/W) double-emulsion drops are prepared to contain gold nanoparticles and hydrogel precursors in innermost drop. Under hypertonic condition, water is selectively pumped out from the innermost drops. Therefore, gold nanoparticles are gently concentrated without forming aggregates, which are then captured by hydrogel matrix. The resulting microgels have a concentration of gold nanoparticles ≈30 times higher and show Raman intensity two orders of magnitude higher than those with no enrichment. In addition, even distribution of gold nanoparticles results in uniform Raman intensity, providing high signal reproducibility. Moreover, as the matrix of the microgel serves as a molecular filter, large adhesive proteins are rejected, which enables the direct detection of small molecules dissolved in the protein solution. It is believed that this advanced SERS platform is useful for in situ detection of toxic molecules in complex mixtures such as biological fluids, foods, and cosmetics.

Highly Sensitive and Selective Nanogap-Enhanced SERS Sensing Platform

This paper reports a highly sensitive and selective surface-enhanced Raman spectroscopy (SERS) sensing platform. We used a simple fabrication method to generate plasmonic hotspots through a direct maskless plasma etching of a polymer surface and the surface tension-driven assembly of high aspect ratio Ag/polymer nanopillars. These collapsed plasmonic nanopillars produced an enhanced near-field interaction via coupled localized surface plasmon resonance. The high density of the small nanogaps yielded a high plasmonic detection performance, with an average SERS enhancement factor of 1.5 × 10⁷. More importantly, we demonstrated that the encapsulation of plasmonic nanostructures within nanofiltration membranes allowed the selective filtration of small molecules based on the degree of membrane swelling in organic solvents and molecular size. Nanofiltration membrane-encapsulated SERS substrates do not require pretreatments. Therefore, they provide a simple and fast detection of toxic molecules using portable Raman spectroscopy.