Colorimetric Analysis of Transmitted Light Through Plasmonic Paper for Next-Generation Point-of-Care (PoC) Devices

This study identifies the optimal conditions for enhancing the performance of the Color Picker System, a device designed for colorimetric sensing using plasmonic paper. A simulation study was conducted toanalyze the transmittance spectra of plasmonic paper embedded in different mixtures, resulting in a comprehensive color chart that includes the chromatic response as well as the RGB values of transmitted light. The filtering properties of the plasmonic paper were evaluated through colorimetric analysis, combining the transmittance characteristics with the spectrum of different light sources. Optimizing the correlation between these filtering properties and the light source enhances both sensitivity and precision. Arrays of nanoparticles with high absorbance, combined with Cold LED light sources, emerge as ideal components for the device set-up. Among the light sources tested, the White LED uniquely generates a red signal while producing the most significant variations in the green channel. In contrast, the Cold LED and Xenon Arc lamp produce the strongest colorimetric signals in the blue channel. This study provides a deep understanding of the filtering properties of plasmonic paper, opening a new way for the implementation of nanoparticle arrays in colorimetric sensing.

A practical approach to quantitative analytical surface-enhanced Raman spectroscopy

Many of the features of SERS, such as its high sensitivity, molecular specificity and speed of analysis make it attractive as an analytical technique. However, SERS currently remains a specialist technique which has not yet entered the mainstream of analytical chemistry. Therefore, this review draws out the underlying principles for analytical SERS and provides practical tips and tricks for SERS quantitation. The aim is to show the readers how to rationally design their SERS experiments to improve quantitation performance. We begin by introducing the three core components in SERS analysis: (1) the enhancing substrate material, (2) the Raman instrument and (3) the processed data that is used to establish a calibration curve. This is followed by discussion of the analytical figures of merit relevant to SERS. In the following sections each of the three essential components in SERS quantitation and how they affect the quality of the analysis are described in more detail using examples from the literature. Finally, we highlight the current challenges in applying SERS to the analysis of complex real-life samples and briefly introduce the state-of-the-art developments on multifunctional substrates, digital SERS and AI-assisted data processing, which will help SERS rise to the challenge of moving out into routine real-world analysis.

SERS Assays Based on Electrospun Nanofibers: Preparation and Analytical Applications

Surface-enhanced Raman spectroscopy (SERS) is a powerful tool and an up-to-date method of analytical chemistry due to its high sensitivity and fingerprint recognition capabilities. Nowadays SERS due to its label-free detection capabilities is being actively developed in medical fields, for example in the analysis of biologically important substances in different matrixes, for potential on-site detection of toxic substances, food safety, and so on. To get the SERS signal, it is necessary the presence of plasmonic nanostructures in the SERS substrates. Electrospun nanofibers have been an attractive alternative to SERS-platforms due to the diversity of advantages, including ease of preparation, structure flexibility, and others. In this review, we summarized the methods of plasmonic nanostructures incorporating substrate based on electrospun nanofibers. Also, the analytical application of SERS-active electrospun nanofibers with embedded nanostructures focused on biologically significant molecules is observed in detail. Finally, the future outlook in the application of these substrates in bioanalysis as the most promising area in analytical chemistry is presented.

Biomarkers and Detection Platforms for Human Health and Performance Monitoring: A Review

Human health and performance monitoring (HHPM) is imperative to provide information necessary for protecting, sustaining, evaluating, and improving personnel in various occupational sectors, such as industry, academy, sports, recreation, and military. While various commercially wearable sensors are on the market with their capability of "quantitative assessments" on human health, physical, and psychological states, their sensing is mostly based on physical traits, and thus lacks precision in HHPM. Minimally or noninvasive biomarkers detectable from the human body, such as body fluid (e.g., sweat, tear, urine, and interstitial fluid), exhaled breath, and skin surface, can provide abundant additional information to the HHPM. Detecting these biomarkers with novel or existing sensor technologies is emerging as critical human monitoring research. This review provides a broad perspective on the state of the art biosensor technologies for HHPM, including the list of biomarkers and their physiochemical/physical characteristics, fundamental sensing principles, and high-performance sensing transducers. Further, this paper expands to the additional scope on the key technical challenges in applying the current HHPM system to the real field.

Paper-based plasmonic substrates as surface-enhanced Raman scattering spectroscopy platforms for cell culture applications

The engineering of advanced materials capable of mimicking the cellular micro-environment while providing cells with physicochemical cues is central for cell culture applications. In this regard, paper meets key requirements in terms of biocompatibility, hydrophilicity, porosity, mechanical strength, ease of physicochemical modifications, cost, and ease of large-scale production, to be used as a scaffold material for biomedical applications. Most notably, paper has demonstrated the potential to become an attractive alternative to conventional biomaterials for creating two-dimensional (2D) and three-dimensional (3D) biomimetic cell culture models that mimic the features of in vivo tissue environments for improving our understanding of cell behavior (e.g. growth, cell migration, proliferation, differentiation and tumor metastasis) in their natural state. On the other hand, integration of plasmonic nanomaterials (e.g. gold nanoparticles) within the fibrous structure of paper opens the possibility to generate multifunctional scaffolds equipped with biosensing tools for monitoring different cell cues through physicochemical signals. Among different plasmonic based detection techniques, surface-enhanced Raman scattering (SERS) spectroscopy emerged as a highly specific and sensitive optical tool for its extraordinary sensitivity and the ability for multidimensional and accurate molecular identification. Thus, paper-based plasmonic substrates in combination with SERS optical detection represent a powerful future platform for monitoring cell cues during cell culture processes. To this end, in this review, we will describe the different methods for fabricating hybrid paper-plasmonic nanoparticle substrates and their use in combination with SERS spectroscopy for biosensing and, more specifically, in cell culture applications.

Designing Efficient Low-Cost Paper-Based Sensing Plasmonic Nanoplatforms

Paper-based platforms can be a promising choice as portable sensors due to their low-cost and facile fabrication, ease of use, high sensitivity, specificity and flexibility. By combining the qualities of these 3D platforms with the optical properties of gold nanoparticles, it is possible to create efficient nanodevices with desired biosensing functionalities. In this work, we propose a new plasmonic paper-based dual localized surface plasmon resonance–surface-enhanced Raman scattering (LSPR-SERS) nanoplatform with improved detection abilities in terms of high sensitivity, uniformity and reproducibility. Specifically, colloidal gold nanorods (GNRs) with a well-controlled plasmonic response were firstly synthesized and validated as efficient dual LSPR-SERS nanosensors in solution using the p-aminothiophenol (p-ATP) analyte. GNRs were then efficiently immobilized onto the paper via the immersion approach, thus obtaining plasmonic nanoplatforms with a modulated LSPR response. The successful deposition of the nanoparticles onto the cellulose fibers was confirmed by LSPR measurements, which demonstrate the preserved plasmonic response after immobilization, as well as by dark-field microscopy and scanning electron microscopy investigations, which confirm their uniform distribution. Finally, a limit of detection for p-ATP as low as 10⁻¹² M has been achieved by our developed SERS-based paper nanoplatform, proving that our optimized plasmonic paper-based biosensing design could be further considered as an excellent candidate for miniaturized biomedical applications.

Ag/Au Nanoparticle-Loaded Paper-Based Versatile Surface-Enhanced Raman Spectroscopy Substrates for Multiple Explosives Detection

We present a systematic study on the fabrication, characterization of versatile, and low-cost filter paper-based surface-enhanced Raman spectroscopy (SERS) substrates loaded with salt-induced aggregated Ag/Au nanoparticles (NPs). These were demonstrated as efficient SERS substrates for the detection of multiple explosive molecules such as picric acid (5 μM), 2,4-dinitrotoluene (1 μM), and 3-nitro-1,2,4-triazol-5-one (10 μM) along with a common dye molecule (methylene blue, 5 nM). The concentrations of the dye and explosive molecules in terms of mass represent 31.98 pg, 11.45 ng, 1.82 ng, and 13.06 ng, respectively. Silver (Ag) and gold (Au) colloidal NPs were prepared by femtosecond laser (∼50 fs, 800 nm, 1 kHz) ablation of Ag/Au-target immersed in distilled water. Subsequently, the aggregated nanoparticles were achieved by mixing the pure Ag and Au NPs with different concentrations of NaCl. These aggregated NPs were characterized by UV-visible absorption and high-resolution transmission electron microscopy techniques. The SERS substrates were prepared by soaking the filter paper in aggregated NPs. The morphologies of the paper substrates were investigated using field-emission scanning electron microscopy technique. We have achieved superior enhancements with high reproducibility and sensitivity for filter paper substrates loaded with Ag/Au NPs mixed for an optimum concentration of 50 mM NaCl.

Dual-Excitation Nanocellulose Plasmonic Membranes for Molecular and Cellular SERS Detection

We demonstrate that cellulose nanofiber (CNF) biomaterials with high transparency and mechanical robustness can be combined with gold nanorods to form a multifunctional porous membrane for dual-mode surface-enhanced Raman scattering (SERS) detection of both small molecules and cells. The nanoporous nature of the nanofiber membranes allows for effective molecular filtration and preconcentration of the analytes, further boosting the SERS performance. Specifically, because of the low fluorescence and Raman background of the CNF matrix, extremely low loading density of gold nanorods can be used. The nanorod assemblies within the CNF network can be resonantly driven by a 532 nm laser (transverse plasmonic mode) and near resonantly driven at by a 785 nm laser (longitudinal mode), facilitating dual operational modes at two excitation wavelengths. The shorter wavelength excitation mode yields better Raman scattering efficiency and has been demonstrated to be capable of detecting rhodamine 6G (R6G) dyes down to picomolar concentrations. On the other hand, the longer wavelength excitation mode provides autofluorescence suppression for the better detection of microorganisms such as Escherichia coli, shortening the required integration time from hours to minutes. Upon drastically lowering the spectral background noise and utilizing nanofiltration, the plasmonic CNF membranes reported here show significantly improved SERS sensitivity and detection fidelity as compared to traditional metal, metal oxide, synthetic polymer, and paper SERS substrates.

Advancing Peptide-Based Biorecognition Elements for Biosensors Using in-Silico Evolution

Sensors for human health and performance monitoring require biological recognition elements (BREs) at device interfaces for the detection of key molecular biomarkers that are measurable biological state indicators. BREs, including peptides, antibodies, and nucleic acids, bind to biomarkers in the vicinity of the sensor surface to create a signal proportional to the biomarker concentration. The discovery of BREs with the required sensitivity and selectivity to bind biomarkers at low concentrations remains a fundamental challenge. In this study, we describe an in-silico approach to evolve higher sensitivity peptide-based BREs for the detection of cardiac event marker protein troponin I (cTnI) from a previously identified BRE as the parental affinity peptide. The P2 affinity peptide, evolved using our in-silico method, was found to have ∼16-fold higher affinity compared to the parent BRE and ∼10 fM (0.23 pg/mL) limit of detection. The approach described here can be applied towards designing BREs for other biomarkers for human health monitoring.

Metal-Organic Framework Encapsulation for the Preservation and Photothermal Enhancement of Enzyme Activity

Interfacing biomolecules with functional materials is a key strategy toward achieving externally-triggered biological function. The rational integration of functional proteins, such as enzymes, with plasmonic nanostructures that exhibit unique optical properties such as photothermal effect provides a means to externally control the enzyme activity. However, due to the labile nature of enzymes, the photothermal effect of plasmonic nanostructures is mostly utilized for the enhancement of the biocatalytic activity of thermophilic enzymes. In order to extend and utilize the photothermal effect to a broader class of enzymes, a means to stabilize the immobilized active protein is essential. Inspired by biomineralization for the encapsulation of soft tissue within protective exteriors in nature, metal-organic framework is utilized to stabilize the enzyme. This strategy provides an effective route to enhance and externally modulate the biocatalytic activity of enzymes bound to functional nanostructures over a broad range of operating environments that are otherwise hostile to the biomolecules.

Structure-dependent SERS activity of plasmonic nanorattles with built-in electromagnetic hotspots

We investigate the surface enhanced Raman scattering (SERS) activity of plasmonic nanorattles obtained through different degrees of galvanic replacement of Au@Ag nanocubes.