Size-Dependent Electrochemistry of Laser-Induced Graphene Electrodes.

Laser-induced graphene (LIG) electrodes have become popular for electrochemical sensor fabrication due to their simplicity for batch production without the use of reagents. The high surface area and favorable electrocatalytic properties also enable the design of small electrochemical devices while retaining the desired electrochemical performance. In this work, we systematically investigated the effect of LIG working electrode size, from 0.8 mm to 4.0 mm diameter, on their electrochemical properties, since it has been widely assumed that the electrochemistry of LIG electrodes is independent of size above the microelectrode size regime. The background and faradaic current from cyclic voltammetry (CV) of an outer-sphere redox probe [Ru(NH3)6]3+ showed that smaller LIG electrodes had a higher electrode roughness factor and electroactive surface ratio than those of the larger electrodes. Moreover, CV of the surface-sensitive redox probes [Fe(CN)6]3- and dopamine revealed that smaller electrodes exhibited better electrocatalytic properties, with enhanced electron transfer kinetics. Scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy showed that the physical and chemical surface structure were different at the electrode center versus the edges, so the electrochemical properties of the smaller electrodes were improved by having rougher surface and more density of the graphitic edge planes, and more oxide-containing groups, leading to better electrochemistry. The difference could be explained by the different photothermal reaction time from the laser scribing process that causes different stable carbon morphology to form on the polymer surface. Our results give a new insight on relationships between surface structure and electrochemistry of LIG electrodes and are useful for designing miniaturized electrochemical devices.

Sequential Flow Controllable Microfluidic Device for G-Quadruplex DNAzyme-Based Electrochemical Detection of SARS-CoV-2 Using a Pyrrolidinyl Peptide Nucleic Acid.

The coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been a significant health issue globally. Point-of-care (POC) testing that can offer a rapid and accurate diagnosis of SARS-CoV-2 at the early stage of infection is highly desirable to constrain this outbreak, especially in resource-limited settings. Herein, we present a G-quadruplex DNAzyme-based electrochemical assay that is integrated with a sequential flow controllable microfluidic device for the detection of SARS-CoV-2 cDNA. According to the detection principle, a pyrrolidinyl peptide nucleic acid probe is immobilized on a screen-printed graphene electrode for capturing SARS-CoV-2 DNA. The captured DNA subsequently hybridizes with another DNA probe that carries a G-quadruplex DNAzyme as the signaling unit. The G-quadruplex DNAzyme catalyzes the H2O2-mediated oxidation of hydroquinone to benzoquinone that can be detected using square-wave voltammetry to give a signal that corresponds to the target DNA concentration. The assay exhibited high selectivity for SARS-CoV-2 DNA and showed a good experimental detection limit at 30 pM. To enable automation, the DNAzyme-based assay was combined with a capillary-driven microfluidic device featuring a burst valve technology to allow sequential sample and reagent delivery as well as the DNA target hybridization and enzymatic reaction to be operated in a precisely controlled fashion. The developed microfluidic device was successfully applied for the detection of SARS-CoV-2 from nasopharyngeal swab samples. The results were in good agreement with the standard RT-PCR method and could be performed within 20 min. Thus, this platform offers desirable characteristics that make it an alternative POC tool for COVID-19 diagnosis.

Single-step electropolymerization on a printed sensor towards a conductive thin film polymer for the simultaneous determination of drug metabolites: 5-aminosalicylic acid and sulfapyridine.

Amino acid conductive polymers can easily form a thin film on a sensor surface by an electrochemical process. Therefore, we are pioneers in reporting the electropolymerization of L-methionine on the surface of a screen-printed graphene electrode to obtain a disposable electrochemical sensor for determining drug metabolites (5-aminosalicylic acid (5-ASA) and sulfapyridine (SPD)) of sulfasalazine (SSZ) simultaneously. In this work, the developed sensor was facilely created through a single step of electropolymerization under mild conditions (0.1 M phosphate buffer pH 7.0) using cyclic voltammetry. Important parameters in the synthesis process were systematically investigated followed by surface composition and morphology studies. Then, analytical performances, comprising sensitivity, selectivity, stability, reproducibility, and sample preparation, were carefully evaluated. Under optimal conditions, the proposed methodology demonstrated a highly sensitive and selective simultaneous detection of 5-ASA and SPD with wide linear dynamic ranges of 1-50 µM and 80-250 µM and low detection limits of 0.60 and 0.57 µM for 5-ASA and SPD, respectively. To evaluate the potential of the designed sensor, it was successfully applied by simultaneously determining 5-ASA and SPD in real human urine samples on the same day (intra-day study) and on three different days (inter-day study).

Toward the rapid diagnosis of sepsis: dendritic copper nanostructure functionalized diazonium salt modified screen-printed graphene electrode for IL-6 detection.

Sepsis, an infectious disease affecting millions of people's health worldwide each year, calls for urgent attention to an improvement of analytical devices. Chemiluminescence immunoassay is a typical diagnostic method utilized to assess the risk development of sepsis. However, due to its high-cost, delayed, and complicated procedure, the practical utilization is therefore undoubtedly limited, especially for point-of-care test. Herein, we fabricated for the first time an immunosensor based on dendritic copper nanostructures (CuNSs) combined with 4-aminobenzoic acid (4-AB, the diazonium salt) as antibody linker modified on a screen-printed graphene electrode for the early detection of the sepsis biomarker interleukin-6 (IL-6). The electrode fabrication is made by electrodeposition, thus eliminating the multistep of nanomaterial synthesis and time wasting. The resulting dendritic CuNSs significantly increase the effective surface area (1.2 times) and the sensor's performance. The morphology of this combination was characterized using CV, EIS, SEM, EDX, and FTIR techniques. In the detection process, the appearance of IL-6 suppresses the current response of the redox probe indicator measured by differential pulse voltammetry due to the antibody-antigen complex. The subtraction of signal (ΔI) was interpreted as IL-6 concentration. This sensor exhibited a linear range from 0.05 to 500 pg mL-1 with low detection limit of 0.02 pg mL-1, proving a possibility for early sepsis screening. In addition, the established immunosensor can successfully quantify IL-6 in human serum sample, in which the results agreed well with those achieved using the standard approach, further showing high practical applicability of this developed immunosensor.

Large-scale fabrication of ion-selective electrodes for simultaneous detection of Na+, K+, and Ca2+ in biofluids using a smartphone-based potentiometric sensing platform.

A significant bottleneck exists for mass-production of ion-selective electrodes despite recent developments in manufacturing technologies. Here, we present a fully-automated system for large-scale production of ISEs. Three materials, including polyvinyl chloride, polyethylene terephthalate and polyimide, were used as substrates for fabricating ion-selective electrodes (ISEs) using stencil printing, screen-printing and laser engraving, respectively. We compared sensitivities of the ISEs to determine the best material for the fabrication process of the ISEs. The electrode surfaces were modified with various carbon nanomaterials including multi-walled carbon nanotubes, graphene, carbon black, and their mixed suspensions as the intermediate layer to enhance sensitivities of the electrodes. An automated 3D-printed robot was used for the drop-cast procedure during ISE fabrication to eliminate manual steps. The sensor array was optimized, and the detection limits were 10-5 M, 10-5 M and 10-4 M for detection of K+, Na+ and Ca2+ ions, respectively. The sensor array integrated with a portable wireless potentiometer was used to detect K+, Na+ and Ca2+ in real urine and simulated sweat samples and results obtained were in agreement with ICP-OES with good recoveries. The developed sensing platform offers low-cost detection of electrolytes for point-of-care applications.

Integrated Lateral Flow Electrochemical Strip for Leptospirosis Diagnosis.

LipL32 is an outer membrane protein present only on pathogenic Leptospira species, which is the causative agent of leptospirosis. Leptospirosis symptoms are often misdiagnosed with other febrile illnesses as the clinical manifestations are non-specific. Therefore, an accurate diagnostic tool for leptospirosis is indeed critical for proper and prompt treatment. Typical diagnosis via serological assays is generally performed to assess the antibodies produced against Leptospira. However, their delayed antibody response and complicated procedure undoubtedly limit the practical utilization especially in a primary care setting. Here, we demonstrate for the first time an early-stage detection of LipL32 by an integrated lateral-flow immunoassay with an electrochemical readout (eLFIA). A ferrocene trace tag was monitored via differential pulse voltammetry operated on a smartphone-based device, thus allowing for on-field testing. A superior performance in terms of the lowest detectable limit of detection of 8.53 pg/mL and broad linear dynamic range (5 orders of magnitude) among other sensors available thus far was established. Additionally, the developed test strip provided a straightforward yet sensitive approach for diagnosis of leptospirosis using the collected human sera from patients, in which the results were comparable to the real-time polymerase chain reaction technique.

Resistance-Based Lateral Flow Immunosensor with a NFC-Enabled Smartphone for Rapid Diagnosis of Leptospirosis in Clinical Samples.

Leptospirosis is one of the most life-threatening tropical diseases caused by pathogenic Leptospira. To date, a diagnostic device that offers rapid and sensitive detection of leptospires has been still in demand for proper treatment to reduce the mortality rate. Herein, we create a resistance-based lateral flow immunosensor diagnosis device (R-LFI) that integrates near-field communication (NFC) with a portable smartphone for leptospiral detection in clinical samples. A specific monoclonal antibody against the pathogen was coated on a nitrocellulose membrane (NCM) where the test line was collocated. Two electrodes with a sandwich-like configuration were installed employing a conductive double-sided adhesive tape and connected with a NFC smartphone-based detection system. A half-sandwich immunocomplex formation induced high proton conduction, resulting in a considerable decrement in resistive response. The performance of the R-LFI sensor was evaluated using recombinant LipL32 (rLipL32), Leptospira interrogans, and clinical samples. The R-LFI device exhibited linear responses toward rLipL32 protein in phosphate buffer and L. interrogans-spiked healthy human serum samples within the concentration ranging from 1 to 1000 ng mL-1 (limit of detection (LOD): 0.29 ng mL-1) and from 104 to 106 cell mL-1 (LOD: 4.89 × 103 cell mL-1), respectively. Our R-LFI sensor successfully detected L. interrogans-positive clinical samples as confirmed by polymerase chain reaction (PCR). This platform offers high specificity, selectivity, simplicity, miniscule sample volume, and no labeling element requirement. These desirable features make it particularly suitable for countries where medical facilities and resources are limited.

A portable blood lactate sensor with a non-immobilized enzyme for early sepsis diagnosis.

Early determination of blood lactate levels may accelerate the detection of sepsis, one of the most time-sensitive illnesses. We developed and validated a portable blood lactate detection kit for clinical screening that can measure early bedside lactate levels in intensive care unit (ICU) patients suspected of having sepsis. A TiO2 sol-G nanocomposite was prepared and coated on a screen-printed carbon electrode (SPCE) integrated with non-immobilized lactate oxidase (LOx) to produce a novel lactate biosensor with high sensitivity and high storage stability for human blood lactate measurement. The detection kit was based on an electrochemical technique and showed a wide linear range of 1-20 mM (R2 = 0.9937) with a low detection limit of 0.2 mM for lactate detection. This allowed for differentiating patient groups who may have sepsis using a cut-off level of 4 mM. The device was successfully implemented for blood lactate determination in critical patients, showing an accuracy range from 75% to 112%. This device provided high-precision and rapid quantitative information validated using a blood gas analyzer. Our detection kit might help to reduce the morbidity and mortality rates in severe sepsis and septic shock patients in community hospitals.

Electrochemical immunoassay for detection of hepatitis C virus core antigen using electrode modified with Pt-decorated single-walled carbon nanotubes.

Pt nanoparticles deposited on single-walled carbon nanotubes (PtSWCNTs), synthesized via the deposition precipitation (DP) method, were introduced as a substrate for immobilizing antibodies on an electrode surface and then enhancing the electrochemical sensitivity. A PtSWCNT-modified paper-based screen-printed graphene electrode was successfully developed to diagnose hepatitis C virus (HCV) infection. The hepatitis C virus core antigen (HCV-cAg) level was determined by differential pulse voltammetry (DPV) using [Fe(CN)6]3-/4- as a redox solution. In the presence of HCV-cAg, the DPV current response decreased with increasing HCV-cAg concentration. Under the optimal conditions, the change in current response provides a good linear correlation with the logarithm of HCV-cAg concentration in the range 0.05 to 1000 pg mL-1 (RSD < 5%), and the limit of detection was 0.015 pg mL-1 (or 0.71 fmol L-1). Furthermore, the proposed immunosensor has been utilized to quantify HCV-cAg in human serum samples with reliable results compared with standard immunoassays (% relative error < 10%). This sensor offers a simple, sensitive, selective, disposable, and inexpensive means for determination of HCV-cAg in human serum samples. The paper-based label-free immunosensor is versatile and feasible for clinical diagnosis.

A novel delayed lateral flow immunoassay for enhanced detection of SARS-CoV-2 spike antigen.

A new detection strategy was developed to improve the sensitivity of a lateral flow immunoassay platform utilizing a delayed hydrophobic barrier fabricated with trimethylsilyl cellulose (TMSC). The SARS-CoV-2 spike receptor-binding domain (SARS-CoV-2 SP RBD) antigen was chosen as a model analyte to demonstrate the superior detectability of this scheme. The novel device consists of 2 separate layers, so-called delayed lateral flow immunoassay (d-LFIA). The upper layer is intended for the analyte or sample flow path, where the test solution flows freely straight to the detection zone to bind with the primary antibody. The lower layer, located just underneath, is designed for the SARS-CoV-2 spike receptor-binding domain-conjugated gold nanoparticles (SARS-CoV-2 SP RBD-AuNPs) used for producing a colorimetric signal. This layer is fabricated with a TMSC barrier to time-delay the movement of SARS-CoV-2 SP RBD-AuNPs, thus allowing the antigen to bind with the primary antibody more efficiently. This platform exhibited a 2.6-fold enhancement in the sensitivity and 9.1-fold improvement in the limit of detection (LOD) as compared with the conventional LFIA. In addition, this d-LFIA device was satisfactorily applied to accurate screening of COVID-19 patients.

Smartphone-based electrochemical analysis integrated with NFC system for the voltammetric detection of heavy metals using a screen-printed graphene electrode.

The electrochemical determination of five heavy metals is demonstrated using a wireless and card-sized potentiostat coupled with a smartphone through near-field communication (NFC) technology. A smartphone application was customized to command the NFC potentiostat, collect real-time signals, process the data, and ultimately display the quantities of the selected elements. The screen-printed graphene electrode (SPGE) was simply fabricated and modified using different nanomaterials for each heavy metal. Using differential pulse voltammetry (DPV) mode on the smartphone, the signal peaks were presented at + 10 mV for As(III), + 350 mV for Cr(VI), 0 mV for Hg(II), - 900 mV for Cd(II), and - 680 mV vs. Ag/AgCl for Pb(II). The linear ranges were 25-500, 250-25,000, 100-1,500, 25-750, 25-750 ng mL-1 with detection limits of 3.0, 40, 16, 2.0, and 0.95 ng mL-1 for As(III), Cr(VI), Hg(II), Cd(II), and Pb(II), respectively. The reproducibility in terms of relative standard deviation was less than 8.8% (n = 5 devices) of the developed SPGE coupled with the NFC potentiostat. Various samples for different applications (e.g., food safety and environmental monitoring) were analyzed and quantified using the proposed sensors. The results from this sensor indicate that there is no significant difference (95% confidence level) compared with those obtained from the traditional ICP-OES method, while the recoveries were found in the acceptable range of 80-111%. Hence, it can be deduced that this recent advanced technology of the NFC potentiostat developed for heavy metal analysis offers a highly sensitive and selective detection, yet the sensor remains compact, low-cost, and readily accessible to end-users.

Amplification-free DNA Sensor for the One-Step Detection of the Hepatitis B Virus Using an Automated Paper-Based Lateral Flow Electrochemical Device.

Until now, an electrochemical lateral flow assay (eLFA) capable of detecting nucleic acids has remained a challenge and has been scarcely explored because of its complicated multistep nature. Here, we report an automated paper-based eLFA device for the quantitative detection of the hepatitis B virus (HBV)-the major cause of liver cirrhosis and hepatocellular carcinoma (HCC). Using a time-delayed microfluidic strategy fabricated on paper, an automated and precisely sequenced solution transfer was enabled by single sample loading. A gold metallization strategy was employed for the signal-on electrochemical detection of the target DNA. Furthermore, a pyrrolidinyl peptide nucleic acid (so-called "acpcPNA") was used as a probe in this study because it offers higher specificity and yields lower background currents than those of traditional probes. Under optimal conditions, a broad dynamic range (10 pM to 2 µM) with an excellent detection limit (down to 7.23 pM) was achieved. The overall operation can be completed within 7 min of sample loading. The proposed sensor was successfully applied in HBV DNA detection in sera from patients without any amplification step (e.g., PCR) required, thus simplifying the operation further. Additionally, the results obtained from this present device are in accordance with the standard real-time PCR, thus supporting the accuracy of the method.

Toward the Rapid Diagnosis of Sepsis: Detecting Interleukin-6 in Blood Plasma Using Functionalized Screen-Printed Electrodes with a Thermal Detection Methodology.

This paper reports the detection of the inflammatory and sepsis-related biomarker, interleukin-6 (IL-6), in human blood plasma using functionalized screen-printed electrodes (SPEs) in conjunction with a thermal detection methodology, termed heat-transfer method (HTM). SPEs are functionalized with antibodies specific for IL-6 through electrodeposition of a diazonium linking group and N'-ethylcarbodiimide hydrochloride (EDC) coupling, which was tracked through the use of cyclic voltammetry and Raman spectroscopy. The functionalized SPEs are mounted inside an additively manufactured flow cell and connected to the HTM device. We demonstrate the ability to detect IL-6 at clinically relevant concentrations in PBS buffer (pH = 7.4) with no significant interference from the similarly sized sepsis-related biomarker procalcitonin (PCT). The limit of detection (3σ) of the system is calculated to correspond to 3.4 ± 0.2 pg mL-1 with a working range spanning the physiologically relevant concentration levels in both healthy individuals and patients with sepsis, indicating the sensitivity of the sensor is suitable for the application. Further experiments helped provide a proof-of-application through the detection of IL-6 in blood plasma with no significant interference observed from PCT or the constituents of the medium. Due to the selectivity, sensitivity, straightforward operation, and low cost of production, this sensor platform has the potential for use as a traffic light sensor for the multidetection of inflammatory biomarkers for the diagnosis of sepsis and other conditions in which the rapid testing of blood biomarkers has vital clinical application.

Impedimetric determination of cortisol using screen-printed electrode with aptamer-modified magnetic beads.

A non-invasive aptamer-based electrochemical biosensor using disposable screen-printed graphene electrodes (SPGEs) was developed for simple, rapid, and sensitive determination of cortisol levels. Selective detection of cortisol based on a label-free electrochemical assay was achieved by specific recognition of the cortisol DNA aptamer (CApt). The CApt was modified with streptavidin magnetic beads (MBs) before simple immobilization onto the electrode surface using a neodymium magnet. The electrochemical behavior of the aptamer-based biosensor was assessed by using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) (vs Ag/AgCl). The specific binding between cortisol and CApt resulted in a decrease in charge transfer resistance (Rct) from EIS using [Fe(CN)6]3-/4- with increasing cortisol concentration. Under optimal conditions, a linear range from 0.10 to 100 ng/mL with a low detection limit (3SD/slope) of 2.1 pg/mL was obtained. Furthermore, the proposed biosensing system exhibited a satisfactory recovery in the range 97.4-109.2% with 5.7-6.6% RSD in spiked artificial human sweat. Regarding the applications of this tool, the aptamer-based biosensor has potential to be a versatile and point-of-care (POC) device for simple, sensitive, selective, disposable, and low-cost cortisol detection.

Laser engraved microapillary pump paper-based microfluidic device for colorimetric and electrochemical detection of salivary thiocyanate.

A microcapillary grooved paper-based analytical device capable of dual-mode sensing (colorimetric and electrochemical detection) was demonstrated for analysis of viscous samples (e.g., human saliva). Herein, a hollow capillary channel was constructed via laser engraved micropatterning functions as a micropump to facilitate viscous fluidic transport, which would otherwise impede analysis on paper devices. Using salivary thiocyanate as a model analyte, the proposed device was found to exhibit a promising sensing ability on paper devices without the need for sample pretreatment or bulky instrumentation, as normally required in conventional methods used for saliva analysis. An extensive linear dynamic range covering detection of salivary thiocyanate for both high and trace level regimes (5 orders of magnitude working range) was collectively achieved using the dual-sensing modes. Under optimal conditions, the limit of detection was 6 µmol L-1 with a RSD of less than 5%. An excellent stability for the µpumpPAD was also observed for over 30 days. Real sample analysis using the proposed device was found to be in line with the standard chromatographic method. Benefitting from simple fabrication and operation, portability, disposability, low sample volume (20 µL), and low cost (< 1 USD), the µpumpPAD is an exceptional alternative tool for the detection of various biomarkers in saliva specimens.

Cost-effective paper-based electrochemical immunosensor using a label-free assay for sensitive detection of ferritin.

Ferritin, a blood cell protein containing iron, is a crucial biomarker that is used to estimate the risk assessment of iron deficiency anemia. For point-of-care analysis, a reliable, cost-effective, selective, sensitive, and portable tool is extremely necessary. In this study, a label-free electrochemical immunosensor for detecting ferritin using a paper-based analytical device (ePAD) was created. The device pattern was custom designed onto filter paper to successfully fabricate a deliverable immunosensor. Graphene oxide was first modified onto the working electrode using an inkjet printing technique. An activation step of the electrode surface was then performed using standard 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)/N-hydroxysulfosuccinimide (sulfo-NHS) chemistry. Anti-ferritin antibodies were covalently immobilized onto the amine-reactive ester surface. The amount of ferritin was monitored by observing the electrochemical signal of the selected redox couple by differential pulse voltammetry (DPV). In the presence of ferritin, the sensor showed a considerable decrease in electrochemical response in a concentration-dependent manner. In contrast, there was no observable change in current response detected in the absence of ferritin. The current response provided a good correlation with ferritin concentrations in the range of 1 to 1000 ng mL-1, and the limit of detection (3SD/slope) was found to be 0.19 ng mL-1. This fabricated immunosensor offered good selectivity, reproducibility, and long-term storage stability. In addition, this proposed immunosensor was successfully applied to detect ferritin in human serum with satisfactory results. The promising results suggested that this handmade paper-based immunosensor may be an alternative device for the diagnosis of iron deficiency anemia.

3D paper-based microfluidic device: a novel dual-detection platform of bisphenol A.

A novel platform of microfluidic paper-based analytical devices (µPADs) for dual detection of bisphenol A (BPA), a model analyte, was fabricated using an electronic digital plotter to create the stacked layer of µPADs and generate the lateral-flow channel without using an external pump. Two detection techniques, including electrochemical detection and laser desorption ionization mass spectrometric detection (LDI-MS), were used complementarily to improve the precision in the detection of BPA. The fluid sample was delivered to both detection zones by the capillary action, automatically generated from the fabricated microfluidic device. For an electrochemical sensor, two ballpoint pens filled with silver nanoparticles (AgNPs) and multiwall carbon nanotube (MWCNT) ink were used to print onto the paper with a contact printing method using a digital plotter. To further improve the sensitivity, zinc oxide (ZnO) was used to modify both electrochemical and LDI-MS detection zones. For BPA detection, high electrocatalytic properties and strong UV absorption of ZnO promote the electron transfer in the electrochemical sensor and ionization efficiency in LDI-MS with low interferences compared with a conventional organic matrix. Under optimal conditions, this platform showed a dual detection capability for BPA with a detection limit of 0.35 µM for electrochemical detection and with an ultralow detection limit of 0.01 pM for LDI-MS. This novel platform might be very useful for trace analyses requiring high precision detection of various analytes.

Wide electrochemical window of screen-printed electrode for determination of rapamycin using ionic liquid/graphene composites.

A disposable screen-printed carbon electrode (SPCE) modified with an ionic liquid/graphene composite (IL/G) exhibits a wider potential window, excellent conductivity, and specific surface area for the improvement in the voltammetric signal of rapamycin detection. The modified composite was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). The electrochemical behavior of rapamycin at the modified SPCE was investigated by cyclic and square wave voltammetry in 60:40 EtOH: 0.1 M LiClO4 at pH 5.0. A high reproducible and well-defined peak with a high peak current were obtained for rapamycin detection at a position potential of + 0.98 V versus Ag/AgCl. Under the optimized conditions, the rapamycin concentration in the range 0.1 to 100 µM (R2 = 0.9986) had a good linear relation with the peak current. The detection limit of this method was 0.03 µM (3SD/slope). The proposed device can selectively detect rapamycin in the presence of commonly interfering compounds. Finally, the proposed method was successfully applied to determine rapamycin in urine and blood samples with excellent recoveries. These devices are disposable and cost-effective and might be used as an alternative tool for detecting rapamycin in biological samples and other biological compounds. Graphical abstract Schematic presentation of wide electrochemical window and disposable screen-printed sensor using ionic liquid/graphene composite for the determination of rapamycin. This composite can enhance the oxidation current and expand the potential for rapamycin detection.

An origami paper-based electrochemical immunoassay for the C-reactive protein using a screen-printed carbon electrode modified with graphene and gold nanoparticles.

An origami paper-based electrochemical immunoassay for C-reactive protein (CRP) detection is described. The assay integrates multiple steps of electrode modification into a single device. A graphene-modified screen-printed carbon electrode (G/SPCE) was employed to enhance sensitivity. Gold nanoparticles were first electrodeposited onto the G/SPCE, followed by a self-assembled monolayer of L-cysteine. The capture anti-CRP was then covalently immobilized on the modified electrode. CRP was quantified by measuring the changes in the charge-transfer resistance of the electrode by using hexacyanoferrate as the redox probe. Cyclic voltammetry and scanning electron microscopy were also applied to verify the successful modification of the electrode. Under optimal conditions, impedance increase in the 0.05-100 µg mL-1 CRP concentration range, and the limit of detection is 15 ng mL-1 (at S/N = 3). The immunoassay was successfully applied to the determination of CRP in a certified human serum sample. This method is simple, low-cost, portable and disposable. Graphical abstract An origami paper-based analytical device (oPAD) is described that integrates the multistep of electrode modification, immobilization and detection into a single device. The direct conjugation between the capture antibody and target molecule was allowed to use in this system. The C-reactive protein (CRP) concentration in serum samples was determined using electrochemical impedance spectroscopy.

A nanocomposite prepared from platinum particles, polyaniline and a Ti3C2 MXene for amperometric sensing of hydrogen peroxide and lactate.

A nanocomposite consisting of platinum particles, polyaniline and Ti3C2 MXene (Pt/PANI/MXene) was used to modify a screen-printed carbon electrode (SPCE) to obtain sensors for hydrogen peroxide and lactate. This nanocomposite was characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and X-ray powder diffraction (XRD) to determine the physical morphologies and the nanocomposite elements. The modified electrode exhibited the improved current response towards hydrogen peroxide (H2O2) compared with an unmodified electrode and provided a low detection limit of 1.0 µM. When lactate oxidase was immobilized on the modified electrode, the electrode responded to lactate via the H2O2 generated in the enzymatic reaction. The lactate assay was performed by amperometry at a constant potential of +0.3 V (vs. Ag/AgCl). The linear range was found to be from 0.005 to 5.0 mM with a detection limit of 5.0 µM for lactate. Ultimately, this biosensor was used for the determination of lactate in milk samples with high stability and reliability. Graphical abstractSchematic representation of a novel platinum particles/polyaniline/MXene nanocomposite (Pt/PANI/MXene) for screen-printed carbon electrode (SPCE) modification to enhance the specific surface area for immobilization of lactate oxidase (LOx) and use as enzymatic biosensor for lactate determination in milk sample.