Durable nonenzymatic electrochemical sensing using silver decorated multi-walled carbon nanotubes for uric acid detection.

In this study, we demonstrate a facile, durable, and inexpensive technique of producing silver nanoparticles-decorated multi-walled carbon nanotubes (MWCNT/AgNP) on the easy-to-use screen-printed carbon electrodes (SPCE) for non-enzymatic detection of uric acid (UA) in an electrochemical sensor. The developed sensors show great durability for three months in storage, and high specificity performance for preclinical study using spiked UA in a synthetic urine sample. A simple route for this hybrid nanocomposite was proposed through an oxidation-reduction with reflux (ORR) process. A significant increase in the electroactive surface area of SPCE was achieved by modifying it with MWCNT/AgNP. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), Fourier-transform infrared (FT-IR) spectroscopy, Raman spectroscopy, and X-ray diffraction (XRD) analysis confirmed this synthesis. The nanocomposite nanostructure electrodes achieved an outstanding UA detection with a sensitivity of 0.1021 µA/µM and a wide dynamic range of 10-1000 µM. In PBS, the measurements achieved a detection limit of 84.04 nM while in pure synthetic urine; it was 6.074 µM. The constructed sensor exhibits excellent stability and durability for several months, and great specificity against interfering compounds, including dopamine (DA), urea, and glucose. Overall, the present outcomes denote the potential of MWCNT/AgNP-decorated SPCE for early uric acid diagnostics tools in health monitoring.

Miniaturized Microscale Solid-Phase Extraction-Based Module for Highly Efficient DNA Extraction and Purification from Grapevine Samples.

Several developments over the last few years are being directed toward improving DNA-based analysis to simplify, miniaturize, and reduce the time and cost of analysis, with the objective to allow its use in decentralized settings. One of the most interesting fields is DNA extraction and purification, a key step for ensuring good analytical performance. In this sense, microscale solid phase extraction (µSPE) offers paramount advantages for an improved DNA yield. In this work, we have developed a miniaturized module for DNA purification based on µSPE using a borosilicate glass microfiber filter as the solid phase. We also established a protocol for highly efficient DNA purification from vegetable samples, including leaves and grapes from four different varieties from the PDO Douro and two varieties from the Minho wine regions. The protocol demonstrated excellent performance when compared with a commercial kit with a DNA recovery yield of around 50%.

Integrated Approach from Sample-to-Answer for Grapevine Varietal Identification on a Portable Graphene Sensor Chip.

Identifying grape varieties in wine, related products, and raw materials is of great interest for enology and to ensure its authenticity. However, these matrices' complexity and low DNA content make this analysis particularly challenging. Integrating DNA analysis with 2D materials, such as graphene, offers an advantageous pathway toward ultrasensitive DNA detection. Here, we show that monolayer graphene provides an optimal test bed for nucleic acid detection with single-base resolution. Graphene's ultrathinness creates a large surface area with quantum confinement in the perpendicular direction that, upon functionalization, provides multiple sites for DNA immobilization and efficient detection. Its highly conjugated electronic structure, high carrier mobility, zero-energy band gap with the associated gating effect, and chemical inertness explain graphene's superior performance. For the first time, we present a DNA-based analytic tool for grapevine varietal discrimination using an integrated portable biosensor based on a monolayer graphene field-effect transistor array. The system comprises a wafer-scale fabricated graphene chip operated under liquid gating and connected to a miniaturized electronic readout. The platform can distinguish closely related grapevine varieties, thanks to specific DNA probes immobilized on the sensor, demonstrating high specificity even for discriminating single-nucleotide polymorphisms, which is hard to achieve with a classical end-point polymerase chain reaction or quantitative polymerase chain reaction. The sensor was operated in ultralow DNA concentrations, with a dynamic range of 1 aM to 0.1 nM and an attomolar detection limit of ∼0.19 aM. The reported biosensor provides a promising way toward developing decentralized analytical tools for tracking wine authenticity at different points of the food value chain, enabling data transmission and contributing to the digitalization of the agro-food industry.

Attomolar detection of hepatitis C virus core protein powered by molecular antenna-like effect in a graphene field-effect aptasensor.

Biosensors based on graphene field-effect transistors have become a promising tool for detecting a broad range of analytes. However, their performance is substantially affected by the functionalization protocol. In this work, we use a controlled in-vacuum physical method for the covalent functionalization of graphene to construct ultrasensitive aptamer-based biosensors (aptasensors) able to detect hepatitis C virus core protein. These devices are highly specific and robust, achieving attomolar detection of the viral protein in human blood plasma. Such an improved sensitivity is rationalized by theoretical calculations showing that induced polarization at the graphene interface, caused by the proximity of covalently bound molecular probe, modulates the charge balance at the graphene/aptamer interface. This charge balance causes a net shift of the Dirac cone providing enhanced sensitivity for the attomolar detection of the target proteins. Such an unexpected effect paves the way for using this kind of graphene-based functionalized platforms for ultrasensitive and real-time diagnostics of different diseases.

Facile and controllable synthesis of monodisperse gold nanoparticle bipyramid for electrochemical dopamine sensor.

We demonstrated potential features of gold nanoparticle bipyramid (AuNB) for an electrochemical biosensor. The facile synthesis method and controllable shape and size of the AuNB are achieved through the optimization of cetyltrimethylammonium chloride (CTAC) surfactant over citric acid (CA) ratio determining the control of typically spherical Au seed size and its transition into a penta-twinned crystal structure. We observe that the optimized ratio of CTAC and CA facilitates flocculation control in which Au seeds with size as tiny as ∼14.8 nm could be attained and finally transformed into AuNB structures with an average length of ∼55 nm with high reproducibility. To improve the electrochemical sensing performance of a screen-printed carbon electrode, surface modification with AuNB via distinctive linking procedures effectively enhanced the electroactive surface area by 40%. Carried out for the detection of dopamine, a neurotransmitter frequently linked to the risk of Parkinson's, Alzheimer's, and Huntington's diseases, the AuNB decorated-carbon electrode shows outstanding electrocatalytic activity that improves sensing performance, including high sensitivity, low detection limit, wide dynamic range, high selectivity against different analytes, such as ascorbic acid, uric acid and urea, and excellent reproducibility.

Versatile and Low-Cost Fabrication of Modular Lock-and-Key Microfluidics for Integrated Connector Mixer Using a Stereolithography 3D Printing.

We present a low-cost and simple method to fabricate a novel lock-and-key mixer microfluidics using an economic stereolithography (SLA) three-dimensional (3D) printer, which costs less than USD 400 for the investment. The proposed study is promising for a high throughput fabrication module, typically limited by conventional microfluidics fabrications, such as photolithography and polymer-casting methods. We demonstrate the novel modular lock-and-key mixer for the connector and its chamber modules with optimized parameters, such as exposure condition and printing orientation. In addition, the optimization of post-processing was performed to investigate the reliability of the fabricated hollow structures, which are fundamental to creating a fluidic channel or chamber. We found out that by using an inexpensive 3D printer, the fabricated resolution can be pushed down to 850 µm and 550 µm size for squared- and circled-shapes, respectively, by the gradual hollow structure, applying vertical printing orientation. These strategies opened up the possibility of developing straightforward microfluidics platforms that could replace conventional microfluidics mold fabrication methods, such as photolithography and milling, which are costly and time consuming. Considerably cheap commercial resin and its tiny volume employed for a single printing procedure significantly cut down the estimated fabrication cost to less than 50 cents USD/module. The simulation study unravels the prominent properties of the fabricated devices for biological fluid mixers, such as PBS, urine and plasma blood. This study is eminently prospective toward microfluidics application in clinical biosensing, where disposable, low-cost, high-throughput, and reproducible chips are highly required.

Influence of the Electrolyte Salt Concentration on DNA Detection with Graphene Transistors.

Liquid-gated Graphene Field-Effect Transistors (GFET) are ultrasensitive bio-detection platforms carrying out the graphene's exceptional intrinsic functionalities. Buffer and dilution factor are prevalent strategies towards the optimum performance of the GFETs. However, beyond the Debye length (λD), the role of the graphene-electrolytes' ionic species interactions on the DNA behavior at the nanoscale interface is complicated. We studied the characteristics of the GFETs under different ionic strength, pH, and electrolyte type, e.g., phosphate buffer (PB), and phosphate buffer saline (PBS), in an automatic portable built-in system. The electrostatic gating and charge transfer phenomena were inferred from the field-effect measurements of the Dirac point position in single-layer graphene (SLG) transistors transfer curves. Results denote that λD is not the main factor governing the effective nanoscale screening environment. We observed that the longer λD was not the determining characteristic for sensitivity increment and limit of detection (LoD) as demonstrated by different types and ionic strengths of measuring buffers. In the DNA hybridization study, our findings show the role of the additional salts present in PBS, as compared to PB, in increasing graphene electron mobility, electrostatic shielding, intermolecular forces and DNA adsorption kinetics leading to an improved sensitivity.

Gold nanoparticle-assisted plasmonic enhancement for DNA detection on a graphene-based portable surface plasmon resonance sensor.

The impact of different gold nanoparticle (GNP) structures on plasmonic enhancement for DNA detection is investigated on a few-layer graphene (FLG) surface plasmon resonance (SPR) sensor. Two distinct structures of gold nano-urchins (GNu) and gold nanorods (GNr) were used to bind the uniquely designed single-stranded probe DNA (ssDNA) of Mycobacterium tuberculosis complex DNA. The two types of GNP-ssDNA mixture were adsorbed onto the FLG-coated SPR sensor through the π-π stacking force between the ssDNA and the graphene layer. In the presence of complementary single-stranded DNA, the hybridization process took place and gradually removed the probes from the graphene surface. From SPR sensor preparation, the annealing process of the Au layer of the SPR sensor effectively enhanced the FLG coverage leading to a higher load of the probe DNA onto the sensing interface. The FLG was shown to be effective in providing a larger surface area for biomolecular capture due to its roughness. Carried out in the DNA hybridization study with the SPR sensor, GNu, with its rough and spiky structures, significantly reinforced the overall DNA hybridization signal compared with GNr with smooth superficies, especially in capturing the probe DNA. The DNA hybridization detection assisted by GNu reached the femtomolar range limit of detection. An optical simulation validated the extreme plasmonic field enhancement at the tip of the GNu spicules. The overall integrated approach of the graphene-based SPR sensor and GNu-assisted DNA detection provided the proof-of-concept for the possibility of tuberculosis disease screening using a low-cost and portable system to be potentially applied in remote or third-world countries.

Facile Bacterial Cellulose Nanofibrillation for the Development of a Plasmonic Paper Sensor.

In this present work, a plasmonic sensor is developed through an extremely cheap cellulose-based source, widely known as a food product, nata de coco (NDC). Capturing its interesting features, such as innate surface roughness from naturally grown cellulose during its fermentation period, the engineering and modulation of NDC fibril size and properties were attempted through a high-pressure homogenization (HPH) treatment to obtain highly dense nanofibrils. After the transformation into a thin, paper-sheet form through a casting process, the homogenized bacterial cellulose (HBC) resulting from HPH was compared with the normally agitated bacterial cellulose (BC) pulp and decorated with silver nanoparticles (AgNPs) to produce plasmonic papers, for further application as surface-enhanced Raman scattering (SERS) substrate. As demonstrated in the measurement of Rhodamine 6G (R6G) molecule, the plasmonic HBC paper sheet provided more prominent SERS signals than the plasmonic BC due to its high surface roughness and improved textural properties from the nanofibrillation process favoring better adsorption of AgNPs and effective SERS hotspots generation. The plasmonic HBC obtained a 2 order higher estimated SERS enhancement factor over the plasmonic BC with a limit of detection of approximately 92 fM. Results denote that the proposed approach provides a new, green-synthesis route toward the exploration of biodegradable sources integrated into an inexpensive and simple nanostructuring process for the production of flexible, paper-based, plasmonic sensors.

Plasmonic nanomaterial structuring for SERS enhancement.

Unique structures of a gold island over nanospheres (AuIoN) featuring a three-dimensional (3D) nanostructure on a highly ordered two-dimensional (2D) array of nanospherical particles with different adhesion layers were fabricated as surface-enhanced Raman scattering (SERS) substrates. Ultra-thin Au was thermally evaporated onto PS nanospheres while aluminum oxide (Al2O3) was applied as an Au adhesion layer. The outcomes demonstrate that the higher metallic particle density and surface roughness supplied by the Al2O3 provided larger interatomic bonding than a conventional adhesion layer, the highly-dispersive Cr. Nanosphere lithography (NSL) to deposit templating particles as small as ∼100 nm successfully created a simple initial roughening process which in turn boosted the localized surface plasmon resonance (LSPR) efficiency. So far, PS template deposition of a size less than 200 nm has been challenging, but here, through the use of a simple solvent ratio adjustment on drop-casting NSL, the novelty of natural lithography with downscaled properties as an alternative to the complexity of photolithography which is mostly conducted in the strict ambience of a clean room, is presented. SERS activity was primarily attributed to the synergistic effect of collective LSPRs from the AuIoN structure reinforcing the electromagnetic field, particularly in the crevices of two neighboring AuIoNs, as simulated by FDTD (Finite-Difference Time-Domain) computation. An AuIoN fabricated by the integration of Al2O3 with thinner Au particles showed the optimum SERS activities with an improved enhancement factor of 1.51 × 106. Overall, a non-lithographic technique in tuning SERS hotspots and favorable characteristics of Al2O3 for ultra-thin Au adhesion support, which can potentially be used in the fabrication of various devices, was demonstrated.

Surface Plasmon Resonance Optical Sensor: A Review on Light Source Technology.

The notion of surface plasmon resonance (SPR) sensor research emerged more than eight decades ago from the first observed phenomena in 1902 until the first introduced principles for gas sensing and biosensing in 1983. The sensing platform has been hand-in-hand with the plethora of sensing technology advancement including nanostructuring, optical technology, fluidic technology, and light source technology, which contribute to substantial progress in SPR sensor evolution. Nevertheless, the commercial products of SPR sensors in the market still require high-cost investment, component, and operation, leading to unaffordability for their implementation in a low-cost point of care (PoC) or laboratories. In this article, we present a comprehensive review of SPR sensor development including the state of the art from a perspective of light source technology trends. Based on our review, the trend of SPR sensor configurations, as well as its methodology and optical designs are strongly influenced by the development of light source technology as a critical component. These simultaneously offer new underlying principles of SPR sensor towards miniaturization, portability, and disposability features. The low-cost solid-state light source technology, such as laser diode, light-emitting diode (LED), organic light emitting diode (OLED) and smartphone display have been reported as proof of concept for the future of low-cost SPR sensor platforms. Finally, this review provides a comprehensive overview, particularly for SPR sensor designers, including emerging engineers or experts in this field.

Effect of Pulsed Light Irradiation on Bioactive, Nonvolatile Components and Antioxidant Properties of Caterpillar Medicinal Mushroom Cordyceps militaris (Ascomycetes).

The caterpillar medicinal mushroom Cordyceps militaris contains many bioactive components, such as adenosine, cordycepin, and polysaccharides. In this study, C. militaris was exposed to 0, 3, 6, or 9 pulses of light irradiation to estimate changes in vitamin D2, bioactive compounds, nonvolatile taste components, and antioxidant properties. In addition, we compared the components and properties of C. militaris mycelia and solid waste medium that had been treated with pulsed light (PL) irradiation. Overall, PL irradiation of C. militaris increased the vitamin D2 content and increased the total amino acid levels 9-48%; the antioxidant properties of the mycelia treated with 0 pulses and of the solid waste medium treated with 3 pulses all exhibited lower half-maximal effective concentrations. Therefore, PL irradiation affected the amounts of bioactive compounds, but the irradiated samples still contained intense umami taste and a sufficient amount of antioxidant components.

Spin-coated Au-nanohole arrays engineered by nanosphere lithography for a Staphylococcus aureus 16S rRNA electrochemical sensor.

The nanopatterning of gold nanoparticle (AuNP) arrays on an indium tin oxide (ITO) electrode using efficient and low-cost methods is described. This process used nanosphere lithography (NSL) encompassing the deposition of monolayered Polystyrene (PS) followed by a convective self-assembly drop coating protocol onto the ITO substrate that further acted as the mask after the AuNP assembly. The results showed that spin-coating allowed AuNPs to follow the contour and adhere to the PS nanospheres. The final products, after etching the PS, generated a highly ordered Au-nanohole array on an ITO substrate. The Au-nanohole arrays on the ITO electrode provided a greater surface area and successfully enhanced the peak current of electrochemical measurements by 82% compared with bare ITO and was used to detect Staphylococcus aureus 16S rRNA hybridization. In contrast to non-templated AuNP structures, the Au-nanohole arrays on the ITO electrode contributed to an optimum sensitivity improvement in DNA hybridization detection by 23%, along with an impressive limit of detection (LOD) of 10 pM. The high specificity of this distinguished structure was also achieved in the hybridization measurements of multi-analyte pathogens. These findings indicate that the combination of PS nanosphere lithography, followed by the spin-coating of AuNPs, leads to an inexpensive and simple engineering process that effectively generates uniform Au-nanohole arrays on ITO, which provides a greater surface area to optimize the electrochemical performance of the DNA biosensor.

Programming a nonvolatile memory-like sensor for KRAS gene sensing and signal enhancement.

A programmable field effect-based electrolyte-insulator-semiconductor (EIS) sensor constructed with a nonvolatile memory-like structure is proposed for KRAS gene DNA hybridization detection. This programmable EIS structure was fabricated with silicon oxide (SiO2)/silicon nitride (Si3N4)/silicon oxide on a p-type silicon wafer, namely electrolyte-oxide-nitride-oxide-Si (EONOS). In this research, voltage stress programming from 4 to 20V was applied to trigger holes confinement in the nitride-trapping layer that, consequently, enhances the DNA attachment onto the sensing surface due to additional electrostatic interaction. Not solely resulting from the higher DNA load, the programming may affect the orientation of the DNA that finally contributes to the change in capacitance. Findings have shown that a higher voltage program is able to increase the total capacitance and results in ~3.5- and ~5.5-times higher sensitivities for a series of concentrations for complementary DNA and wild type versus mutant DNA hybridization detection, respectively. Overall, it has been proven that the voltage program on the nonvolatile memory-like structure of EONOS is a notable candidate for genosensor development, scoping the diagnosis of a single nucleotide polymorphism (SNP)-related disease.