Paper Microfluidics and Tailored Gold Nanoparticles for Nonenzymatic, Colorimetric Multiplex Biomarker Detection

A highly sensitive fluorescence biosensor based on polylysine functionalized quantum dots for serum GDF-15 detection

Growth differentiation factor-15 (GDF-15) is a stress-responsive cytokine that increases in tissue injury and inflammatory states. The circulation level of GDF-15 is firmly correlated with cardiovascular diseases. Herein, we constructed a novel quantum dot-based fluorescent immunosensor for the sensitive detection of serum GDF-15. In this proposed platform, green-emission water-soluble carboxyl-capped CdTe quantum dots were synthesized as fluorescent labels, conjugated with lysine-rich biotinylated peptides P16K to amplify fluorescent signals, and then linked to antibodies via a biotin-streptavidin system to obtain the fluorescent detection probes. The probes were then integrated into a fluorescence-linked immunosorbent assay (FLISA) platform for GDF-15 detection, achieving a wide linear range (6-1600 pg/mL) and low limits of detection (0.98 pg/mL). Moreover, our approach has been demonstrated in clinical validation experiments performed on human serum samples, in which the results obtained were consistent with those from commercial ELISA kits. Due to its higher sensitivity in comparison to commercial ELISA kits, the platform shows excellent potential for early diagnosis and risk screening of cardiovascular diseases.

A dual-functional nanogold tablet as a plasmonic and nanozyme sensor for point-of-care applications

Point-of-care (POC) devices provide on-site disease diagnosis, particularly in resource-limited settings. Despite considerable progress in POC testing, the availability of commercial devices remains limited, primarily due to challenges in detection sensitivity and portability. Furthermore, advancements in existing POC devices are essential to better meet the needs of end-users. Herein, we present a colorimetric dual-functional tablet sensor using dextran-gold nanoparticles (dAuNPs) to detect and quantify uric acid and glucose levels in urine. Our tablet sensor combines the plasmonic and nanozyme properties of dAuNPs, resulting in highly sensitive detection of both biomarkers. Interestingly, we fabricated the nanogold tablet directly from the dAuNP solution without the addition of any external stabilizer or tablet-forming reagent, thus naming it a direct tablet. An enzyme-free approach was employed for uric acid detection, providing a wide detection range of 0.00187-7.8 mM and a low detection limit of 0.0037 mM, attributed to the hydrogen bonding between dextran and uric acid. On the other hand, the unique nanozyme properties of dAuNPs exhibited exclusive POx-mimetic activity for glucose detection (K m = 0.106 mM and V max = 369.72 mM min-1), with a lower detection limit of 0.625 mM. Our dual-functional tablet offers exceptional substrate selectivity for the colorimetric-chromogenic assay of both uric acid and glucose. This dual-functionality not only provides a highly sensitive, selective, and cost-effective detection strategy for resource-limited settings but also introduces a new avenue for designing customizable plasmonic-nanozyme nanogold tablet sensors as a powerful tool for rapid diagnosis.

TimePAD─Unveiling Temporal Sequence ELISA Signal by Deep Learning for Rapid Readout and Improved Accuracy in a Microfluidic Paper-Based Analytical Platform

The integration of paper-based microfluidics with deep learning represents a pivotal trend in enhancing diagnostic capabilities. This paper introduces a new approach to improve the performance of a paper-based microfluidic enzyme-linked immunosorbent assay (ELISA) by training the temporal sequence colorimetric data rather than static data conventionally, using deep learning. Traditional deep learning-assisted ELISA analysis methods usually rely on a single snapshot of the reaction at its end, which limits the further improvement of sensitivity and specificity (or accuracy for combined evaluation), as it misses dynamic changes in the reaction over time. In this work, we developed a temporal sequence-enhanced paper analytical device (TimePAD) that captures continuous video data of the ELISA reaction, which contains the dynamic colorimetric changes. With the YOLOv8 deep learning alogrithm and the Rabbit IgG as the model for ELISA assay, we can use the initial 20 min signal instead of waiting for 30 min for full reaction, achieving a 33% reduction in the turnaround time. Moreover, the overall accuracy at 20 min is 94.1%, which is slightly improvement to the 93.5% using a traditional single snapshot method at 30 min. This method not only accelerates result interpretation but also enhances the overall efficiency of diagnostics, making it particularly valuable for time-sensitive point-of-care testing applications. Lastly, to demonstrate its real-world use, we expanded to the disease biomarker cTnI detection and obtained accuracy of 98.1% within only 10 min, compared to 25 min with 97.8% accuracy in traditional methods.

Portable paper-based microfluidic devices with Cu1-xAgxS NPs modification for multiplex intelligent visualized detection of adrenaline and glucose simultaneously

BACKGROUND

Adrenaline and glucose are essential biomarkers in human body for maintaining metabolic balance. Abnormal levels of adrenaline and glucose are associated with various diseases. Therefore, it is important to design portable, on-site devices for rapid adrenaline and glucose analysis to safeguard health. Traditional paper-based analytical devices (μPADs) for multiplexed detection typically required multiple recognized probes, increasing the cost and complexity. Moreover, non-uniform color distribution, caused by lateral chromatographic elution effect, reduced results accuracy. Hence, it is necessary to construct visualized μPADs with controlled flow capability using single sensing probe for simultaneous adrenaline and glucose analysis in point-of-care testing (POCT).

RESULTS

The portable multiplexed 3D-folded μPADs have been developed for the visualized, accurate and simultaneous detection of adrenaline and glucose, employing Ag-doped copper sulfide nanoparticles (Cu1-xAgxS NPs) as multifunctional sensing probe. The μPADs integrated the sampling entrance layer, buffer layer and test layer into a single device through wax printing, allowing the analyte solution to flow vertically from buffer zones to detection zones, thereby minimizing lateral chromatographic elution effect. Cu1-xAgxS NPs, exhibiting laccase-like and peroxidase-like multi-enzyme activities, were modified on various detection zones of μPADs for multiplex colorimetric analysis of adrenaline and glucose. Based on Cu1-xAgxS-induced colorimetric reactions, the orange-red and blue colors were generated in adrenaline and glucose detection zones, respectively. Subsequently, color signals were converted into RGB values via smartphone application (APP) for rapid and intelligent results analysis. The Cu1-xAgxS-incorporated μPADs exhibited colorimetric uniformity and accuracy with limit of detection (LOD) of 10.2 nM and 11.5 μM, respectively.

SIGNIFICANCE

The adjustable laccase-like and peroxidase-like activities of Cu1-xAgxS NPs induced by Ag doping provide a new perspective for multiple biomarkers analysis. Benefiting from the excellent multienzyme activity of Cu1-xAgxS NPs, μPADs can perform on-site, visual and multiplex analysis using a single probe, demonstrating their potential for POCT in personal healthcare. The 3D folded Cu1-xAgxS-incorporated μPADs with integrated horizontal and vertical flows exhibit uniform color signals for obtaining accurate detection results.

One-step copper deposition-induced signal amplification for multiplex bacterial infection diagnosis on a lateral flow immunoassay device

The lateral flow immunoassay (LFIA) is predominant in rapid diagnostic tests owing to its cost-effectiveness and operational simplicity. However, the conventional LFIA exhibits limited sensitivity and is susceptible to human variance for the result readout, impacting result interpretation. In this study, we introduced a novel one-step copper deposition-induced signal amplification lateral flow immunoassay (osa-LFIA) that markedly enhances the detection sensitivity for Staphylococcus aureus (protein A) and Pseudomonas aeruginosa (exotoxin A). Utilizing gold nanoparticles (AuNPs) as a catalyst, this approach employs ascorbic acid to reduce Cu2+ to Cu0, depositing on AuNPs at the test line and amplifying the signal. A user-friendly design features a three-dimensional paper structure incorporating pre-dried reagents, enabling a streamlined, efficient testing process. The osa-LFIA significantly lowers detection limits to 3 ng mL-1 for protein A and 10 ng mL-1 for exotoxin A, offering a tenfold improvement over conventional LFIA. Additionally, we developed a portable grayscale detection device, achieving less than 10% error in quantitative analysis compared to the data acquired and analyzed in the lab. This entire process, from detection to signal amplification, is completed in just 20 min. For the clinical trial, we utilized the osa-LFIA to test synovial fluid samples infected with Staphylococcus aureus. We also successfully detected different concentrations of the exotoxin A in parallel, with a recovery value of 96%-110%. Our findings demonstrate the osa-LFIA's potential as a rapid, highly sensitive, and simple-to-use diagnostic tool for detecting various pathogens, significantly advancing the field of rapid diagnostic testing.

A gold nanomaterial-integrated distance-based analytical device for uric acid quantification in human urine samples

In this article, we present the first demonstration of a distance-based paper analytical device (dPAD) for uric acid quantification in human urine samples with instrument-free readout and user-friendliness for the rapid diagnosis and prognosis of various related diseases. By employing gold nanoparticles (AuNPs) as a peroxidase-like nanozyme, our proposed technique eliminates the utilization of horseradish peroxidase (HRP), making the device cost-effective and stable. In our dPAD, uric acid in the sample is oxidized by the uricase enzyme and subsequently catalysed with AuNPs in the sample zone, generating hydroxyl radicals (˙OH). Then, the produced ˙OH reacts with 3,3'-diaminobenzidine (DAB) to form poly DAB (oxDAB), resulting in a coloured distance signal in the detection zone of the dPAD. The variation of the distance of the observed red-brown colour is directly proportional to the uric acid concentration. Our sensor exhibited a linear range from 0.50 to 6.0 mmol L-1 (R2 = 0.9922) with a detection limit (LOD) of 0.25 mmol L-1, covering the clinical range of uric acid in urine. Hence, there is no need for additional sample preparation or dilution. Additionally, this assay is highly selective, with no interferences. We also found that this approach could accurately and precisely determine uric acid in human control samples with the recovery ranging from 99.37 to 100.35 with the highest RSD of 4.05%. Our method is comparable with the use of a commercially available uric acid sensor at a 95% confidence interval. Consequently, the developed dPAD offers numerous advantages such as cost-effectiveness, simplicity, and ease of operation with unskilled individuals. Furthermore, this concept can be applied for extensive biosensing applications in monitoring other biomarkers as an alternative analytical point-of-care (POC) device.

Paper immobilized BSA-decorated gold nanoclusters for single-step optical sensing of glucose and cholesterol without cross-reactivity

Minimally invasive methods for detecting glucose, cholesterol and hydrogen peroxide are crucial for monitoring the nutritional and health status of humans and animals. The peroxidase mimetic activity by nanozymes is one of the versatile methods for detecting glucose, cholesterol, hydrogen peroxide, and other biomolecules. However, the strict requirement of acidic pH limits their sensing and interfacing ability with natural enzymes. The present study developed bovine serum albumin (BSA) coated gold nanoclusters (AuNC) immobilized on paper fabric to enable single-step visual detection of glucose, cholesterol and hydrogen peroxide in complex biological fluids like serum and milk. The BSA-AuNC suspension and immobilized paper fabric synergistically interface with the natural oxidative enzymes, glucose oxidase or cholesterol oxidase, at physiological pH. The concomitant loss in the fluorescent intensity of BSA-AuNC-loaded paper fabric exposed to the generated hydrogen peroxide (glucose/glucose oxidase or cholesterol/cholesterol oxidase) was directly proportional to the concentration of glucose or cholesterol. These reactions enabled simple visual detection as well as quantification of hydrogen peroxide, glucose and cholesterol using Image-J software and common smartphone-based mobile applications. The detection ability of BSA-AuNC-embedded paper fabric is specific and remains unaltered in the presence of similar oxidase enzymes or similar substrate analogues. With these unique features, the BSA-AuNC embedded paper fabric stands out as a prominent analytical device with enormous potential as a simple, user-friendly detection tool for monitoring biomolecules that are important to health, nutrition, and environmental safeguarding.

Emerging innovations in portable chemical sensing devices: Advancements from microneedles to hydrogel, microfluidic, and paper-based platforms

With the public heightened emphasis on mitigating the occurrence risks of health-related ailment and optimizing personal physical performance, portable chemical sensing devices emerged as an indispensable component of pervasive health monitoring. Chemical sensing enabled the immediate and on-site identification of biomarkers in biological fluids by integrating colorimetry, fluorescence, electrochemical, and other methods into portable sensor devices. These sensor devices incorporated microneedles, hydrogels, microfluidic modules, and papers, facilitating conformal human-device contact and providing several visual sensing options for disease prevention and healthcare management. This review systematically overviewed recent advancements in chemical sensors for marker detection, categorizing them based on monitoring device types. Furthermore, we also offered recommendations and opportunities for developing portable chemical sensing devices by summarizing sensor integration methods and tracking sites on the human body.

A review: early detection of oral cancer biomarkers using microfluidic colorimetric point-of-care devices

Oral squamous cell carcinoma (OSCC) is the most common type of head and neck cancers. OSCC constitutes 90% of the head and neck malignancies. The delayed identification of oral cancer is the primary cause of ineffective medical treatment. To address this issue, low-cost, reliable point-of-care devices that can be utilized for large-scale screening, even in low-resource settings, including rural areas and primary healthcare centers, are of great interest. Herein, a comprehensive analysis of numerous salivary biomarkers that exhibit significant variations in concentration between individuals with oral cancer and those without is given. Furthermore, the article explores several point-of-care devices that exhibit potential in the realm of oral cancer detection. The biomarkers are discussed with a focus on their structural characteristics and role in oral cancer progression. The devices based on colorimetry and microfluidics are discussed in detail, considering their compliance with the 'REASSURED' criteria given by the World Health Organization (WHO) and suitability for mass screening in low-resource settings. Finally, the discourse revolves around the fundamental aspects pertaining to the advancement of multiplex, cost-effective point-of-care devices designed for widespread screening purposes.

Impedance Characteristics of Microfluidic Channels and Integrated Coplanar Parallel Electrodes as Design Parameters for Whole-Channel Analysis in Organ-on-Chip Micro-Systems

Microfluidics have revolutionized cell culture by allowing for precise physical and chemical environmental control. Coupled with electrodes, microfluidic cell culture can be activated or have its changes sensed in real-time. We used our previously developed reliable and stable microfluidic device for cell growth and monitoring to design, fabricate, and characterize a whole-channel impedance-based sensor and used it to systematically assess the electrical and electrochemical influences of microfluidic channel boundaries coupled with varying electrode sizes, distances, coatings, and cell coverage. Our investigation includes both theoretical and experimental approaches to investigate how design parameters and insulating boundary conditions change impedance characteristics. We examined the system with various solutions using a frequency range of 0.5 Hz to 1 MHz and a modulation voltage of 50 mV. The results show that impedance is directly proportional to electrode distance and inversely proportional to electrode coating, area, and channel size. We also demonstrate that electrode spacing is a dominant factor contributing to impedance. In the end, we summarize all the relationships found and comment on the appropriateness of using this system to investigate barrier cells in blood vessel models and organ-on-a-chip devices. This fundamental study can help in the careful design of microfluidic culture constructs and models that require channel geometries and impedance-based biosensing.

Colorimetric sensing for translational applications: from colorants to mechanisms

Colorimetric sensing offers instant reporting via visible signals. Versus labor-intensive and instrument-dependent detection methods, colorimetric sensors present advantages including short acquisition time, high throughput screening, low cost, portability, and a user-friendly approach. These advantages have driven substantial growth in colorimetric sensors, particularly in point-of-care (POC) diagnostics. Rapid progress in nanotechnology, materials science, microfluidics technology, biomarker discovery, digital technology, and signal pattern analysis has led to a variety of colorimetric reagents and detection mechanisms, which are fundamental to advance colorimetric sensing applications. This review first summarizes the basic components (e.g., color reagents, recognition interactions, and sampling procedures) in the design of a colorimetric sensing system. It then presents the rationale design and typical examples of POC devices, e.g., lateral flow devices, microfluidic paper-based analytical devices, and wearable sensing devices. Two highlighted colorimetric formats are discussed: combinational and activatable systems based on the sensor-array and lock-and-key mechanisms, respectively. Case discussions in colorimetric assays are organized by the analyte identities. Finally, the review presents challenges and perspectives for the design and development of colorimetric detection schemes as well as applications. The goal of this review is to provide a foundational resource for developing colorimetric systems and underscoring the colorants and mechanisms that facilitate the continuing evolution of POC sensors.

Paper-based optical sensor arrays for simultaneous detection of multi-targets in aqueous media: A review

Sensor arrays, which draw inspiration from the mammalian olfactory system, are fundamental concepts in high-throughput analysis based on pattern recognition. Although numerous optical sensor arrays for various targets in aqueous media have demonstrated their diverse applications in a wide range of research fields, practical device platforms for on-site analysis have not been satisfactorily established. The significant limitations of these sensor arrays lie in their solution-based platforms, which require stationary spectrophotometers to record the optical responses in chemical sensing. To address this, this review focuses on paper substrates as device components for solid-state sensor arrays. Paper-based sensor arrays (PSADs) embedded with multiple detection sites having cross-reactivity allow rapid and simultaneous chemical sensing using portable recording apparatuses and powerful data-processing techniques. The applicability of office printing technologies has promoted the realization of PSADs in real-world scenarios, including environmental monitoring, healthcare diagnostics, food safety, and other relevant fields. In this review, we discuss the methodologies of device fabrication and imaging analysis technologies for pattern recognition-driven chemical sensing in aqueous media.

Development of a point-of-care colorimetric metabolomic sensor platform

Metabolomics is the large-scale study of small molecule metabolites within a biological system. It has applications in measuring dietary intake, predicting heart disease risk, and diagnosing cancer. Metabolites are often measured using high-end analytical tools such as mass spectrometers or large spectrophotometers. However, due to their size, cost, and need for skilled operators, using such equipment at the bedside is not practical. To address this issue, we have developed a low-cost, portable, optical color sensor platform for metabolite detection. This platform includes LEDs, sensors, microcontrollers, a power source, and a Bluetooth chip enclosed within a 3D-printed light-tight case. We evaluated the color sensor's performance using both a range of dyed water samples as well as well-established colorimetric reactions for specific metabolite detection. The sensor accurately measured creatinine, L-carnitine, ascorbate, and succinate well within normal human urine levels with accuracy and sensitivity equal to or better than a standard laboratory spectrophotometer. Our color sensor offers a cost-effective, portable alternative for measuring metabolites via colorimetric assays, thereby enabling low-cost, point-of-care metabolite testing.

Hybridization chain reaction assisted multicolor immunosensor for sensitively detection of human chorionic gonadotropin

The level of human chorionic gonadotropin (HCG) is an important indicator for early pregnancy, pregnancy-related diseases trophoblastic diseases and even cancer diagnosis. Therefore, sensitive detection of HCG has crucial significance in clinical, especially in gynaecology and obstetrics. Herein, a hybridization chain reaction (HCR) assisted multicolor immunosensor have been developed for HCG analysis. The proposed method introduced HCR after the immunoreaction between antibody and HCG protein, and produced long double strand DNA (dsDNA) that contain biotin sites. The streptavidin-horseradish peroxidase was linked on the dsDNA by the interaction between biotin and streptavidin, and can further mediated gold nanobipyramids (Au NBPs) etching. The localized surface plasmon resonance absorption peaks of Au NBPs blue shift and accompanied a vivid color change after etching effect. Based on this color change, HCG could be qualitative and semi-quantitative detected. Because of the introduction of HCR and enzyme amplification technique, the proposed method exhibited high sensitivity with a linear range of 0.1-2000 pg/mL and limit of detection (LOD) of 0.1 pg/mL. Finally, the proposed immunosensor was used to detect clinical serum samples. The results show there are no significant differences between clinical results and the test results by this method, indicating the practicability of the proposed method.

Paper-based uric acid assay in whole blood samples by Zn2+ protein precipitation and enzyme-free colorimetric detection

Uric acid (UA) is an important biomarker, as a high concentration in blood can lead to gout and further renal syndrome. Although several point-of-care testing (POCT) devices have been reported to detect UA, there are some limitations such as the requirement for uricase and the complicated pretreatment of serum/plasma samples, which restricts their use at home or in undeveloped areas. In this work, we developed an approach by applying Zn2+ to precipitate proteins and cells in whole blood to avoid interference with the chromogenic reaction. We used carboxymethylcellulose (CMC) to immobilize tetramethylbenzidine (TMB) on a nitrocellulose membrane for colorimetric detection. Using the oxidization properties of H2O2, which turns TMB into oxidized tetramethylbenzidine (TMBox) in the presence of catalyst gold nanoparticles (AuNPs), we successfully constructed an enzyme-free paper-based POCT device using the reduction reaction of UA and TMBox for simple, speedy, and cheap colorimetric detection of UA, achieving a detection time of 8 min, a linear range of 0-150 μg/mL, and an LOD of 25.79 μg/mL. The UA concentration in whole blood samples was further measured and correlated well with the clinical value (R2 = 0.8212). Thus, the proposed assay has the potential for POCT diagnosis, monitoring, and prognosis of diseases related to UA.

The Synergistic Effect on the Mimetic Optical Structure of Feline Eyes toward Household Health Monitoring of Acute and Chronic Diseases

A breakthrough in the performance of bionic optical structures will only be achieved if we can obtain an in-depth understanding of the synergy mechanisms operating in natural optical structures and find ways to imitate them. In this work, inspired by feline eyes, an optical substrate that takes advantage of a synergistic effect that occurs between resonant and reflective structures was designed. The synergistic effect between the reflective and resonant components leads to a Raman enhancement factor (EF) of 1.16 × 107, which is much greater than that achieved using the reflective/resonant cavities on their own. Finite-difference time-domain (FDTD) simulations and experimental results together confirm that the mechanism of this synergistic effect is achieved by realizing multiple reflections and repeated absorptions of light, generating a strong local electric field. Thus, a 2-3 order of magnitude increase in sensitivity could be achieved. More importantly, with the homemade centrifugal device, above optical substrates were further used to develop a rapidly highly sensitive household health monitoring system (detection time <3 min). It can thus be used to give early warning of acute diseases with high risk (e.g., acute myocardial infarction (AMI) and cerebral peduncle). Due to the good reusability and storability (9% and 8% reduction in EF after washing 30 times and 9 months of storage, respectively) of the substrates, the substrates thus reduce detection costs (to ∼$1), making them much cheaper to use than the current gold-standard methods (e.g., ∼$16 for gout detection).

A luminescent MOF-based nonenzymatic probe for colorimetric/photothermal/fluorescence triple-mode assay of uric acid in body fluids

Monitoring the levels of uric acid (UA) in body fluids is of great significance in the clinical diagnosis and therapy of related diseases. Herein, a novel nanocomposite R6G@Fe-MOF based nonenzymatic probe is presented to provide a ratiometric fluorescent, colorimetric, and photothermal triple read-out signal for the visual, sensitive, and convenient assay of UA. The framework structure of the in situ encapsulated R6G@Fe-MOF is found to decompose upon the addition of UA, resulting in the reduction of Fe3+ to Fe2+. This reduction will lead to a rapid increase in fluorescence emission (FL) at 430 nm. Simultaneously, the FL at 573 nm will decrease remarkably due to the inner filter effect (IFE) between UA and R6G@Fe-MOF. Furthermore, the reaction of the generated Fe2+ with potassium ferricyanide (K3 [Fe(CN)6]) can in situ generate Prussian blue (PBNPs) with outstanding color and photothermal properties, which allow for easy colorimetric and photothermal signal readout. The detection limits (LOD) for the colorimetric, fluorometric and photothermal detection are low at 1.68 μM, 0.236 μM, and 1.32 μM respectively. Ultimately, it is successfully employed to determine UA in urine, serum, and saliva, yielding satisfactory results. The constructed R6G@Fe-MOF sensor provides a simple, sensitive, and accurate determination of UA that can be tailored to meet the needs of various applications, and also provides new perspectives for the design and development of versatile sensors for diverse uses.

Phage display selection of a Pb(II) specific peptide and its application as a biorecognition unit for colorimetric detection of Pb(II) ions

Phage display technology employs a library of engineered filamentous M13 viruses infecting only bacteria. In this study, the genuine phage display selection technique was applied to identify a Pb(II) specific peptide. After three rounds of positive selection against Pb(II) coated agarose-based beads and the consecutive negative screenings against interfering metal ions (Al(III), Co(II), Fe(III), Ni(II), and Zn(II)), a final phage library with enhanced Pb(II) binding was obtained. Enzyme Linked Immunosorbent Assay (ELISA) analyses confirmed the selective Pb(II) binding of the enriched viruses. Twenty monoclonal phage plaques were randomly selected, single stranded DNAs (ssDNAs) were isolated and sequenced. Sequencing results revealed four different peptide sequences. Pb9 peptide (KASPYIT) showing the most specific Pb(II) binding was selected for detection studies. Pb9 was synthetically synthesized with additional three cysteine (3xCys) units at C-terminal. Twenty nanometers AuNPs were functionalized with Pb9-3xCys peptides through Au-thiol (Au-S) interaction. A colorimetric Pb(II) detection system was validated using the engineered peptide-AuNP complex at a calculated LOD of around 11 nM (3σ/k, n = 6) for the case study. The detection system was Pb(II) selective over various metal ions (Ag(II), Al(III), Au(III), Cd(II), Co(II), Cr (III), Cu(II), Fe(III), Hg(II), Mg(II), Mn(II), Ni(II), and Zn(II)). Such metal ion specific peptides can be further studied to develop simple, user friendly and cost-effective tools to design alternative detection and bioremediation systems for a circular economy.

Gold Nanomaterial-Based Microfluidic Paper Analytical Device for Simultaneous Quantification of Gram-Negative Bacteria and Nitrite Ions in Water Samples

This study presents a rapid microfluidic paper-based analytical device (μPAD) capable of simultaneously monitoring Gram-negative bacteria and nitrite ions (NO2-) for water quality monitoring. We utilize gold nanoparticles (AuNPs) functionalized with polymyxin molecules (AuNPs@polymyxin) to cause color change due to aggregation for the detection of Gram-negative bacteria, and antiaggregation in the presence of o-phenylenediamine (OPD) for NO2- detection. In this study, Escherichia coli (E. coli) serves as the model of a Gram-negative bacterium. Using the developed μPADs, the color changes resulting from aggregation and antiaggregation reactions are measured using a smartphone application. The linear detection ranges from 5.0 × 102 to 5.0 × 105 CFU/mL (R2 = 0.9961) for E. coli and 0.20 to 2.0 μmol/L (R2 = 0.995) for NO2-. The detection limits were determined as 2.0 × 102 CFU/mL for E. coli and 0.18 μmol/L for NO2-. Notably, the newly developed assay exhibited high selectivity with no interference from Gram-positive bacteria. Additionally, we obtained acceptable recovery for monitoring E. coli and NO2- in drinking water samples with no significant difference between our method and a commercial assay by t test validation. The sensor was also employed for assessing the quality of the pond and environmental water source. Notably, this approach can also be applied to human urine samples with satisfactory accuracy. Furthermore, the assay's stability is extended due to its reliance on AuNPs rather than reagents like antibodies and enzymes, reducing costs and ensuring long-term viability. Our cost-effective μPADs therefore provide a real-time analysis of both contaminants, making them suitable for assessing water quality in resource-limited settings.

Plasmonic Nanoparticle-Enhanced Optical Techniques for Cancer Biomarker Sensing

This review summarizes recent advances in leveraging localized surface plasmon resonance (LSPR) nanotechnology for sensitive cancer biomarker detection. LSPR arising from noble metal nanoparticles under light excitation enables the enhancement of various optical techniques, including surface-enhanced Raman spectroscopy (SERS), dark-field microscopy (DFM), photothermal imaging, and photoacoustic imaging. Nanoparticle engineering strategies are discussed to optimize LSPR for maximum signal amplification. SERS utilizes electromagnetic enhancement from plasmonic nanostructures to boost inherently weak Raman signals, enabling single-molecule sensitivity for detecting proteins, nucleic acids, and exosomes. DFM visualizes LSPR nanoparticles based on scattered light color, allowing for the ultrasensitive detection of cancer cells, microRNAs, and proteins. Photothermal imaging employs LSPR nanoparticles as contrast agents that convert light to heat, producing thermal images that highlight cancerous tissues. Photoacoustic imaging detects ultrasonic waves generated by LSPR nanoparticle photothermal expansion for deep-tissue imaging. The multiplexing capabilities of LSPR techniques and integration with microfluidics and point-of-care devices are reviewed. Remaining challenges, such as toxicity, standardization, and clinical sample analysis, are examined. Overall, LSPR nanotechnology shows tremendous potential for advancing cancer screening, diagnosis, and treatment monitoring through the integration of nanoparticle engineering, optical techniques, and microscale device platforms.

A Facile Method to Fabricate an Enclosed Paper-Based Analytical Device via Double-Sided Patterning for Ionic Contaminant Detection

Microfluidic paper-based analytical devices (μPADs) have been developed for use in a variety of diagnosis and analysis fields. However, conventional μPADs with an open-channel system have limitations for application as analytical platforms mainly because of the evaporation and contamination of the sample solution. This study demonstrates the design and fabrication of an enclosed three-dimensional(3D)-μPAD and its application as a primary early analysis platform for ionic contaminants. To generate the hydrophobic PDMS barrier, double-sided patterning is carried out using a PDMS blade-coated stamp mold that is fabricated using 3D printing. The selective PDMS patterning can be achieved with controlled PDMS permeation of the cellulose substrate using 3D-designed stamp molds. We find the optimal conditions enabling the formation of enclosed channels, including round shape pattern and inter-pattern distance of 10 mm of stamp design, contact time of 0.5 min, and spacer height of 300 µm of double-sided patterning procedure. As a proof of concept, this enclosed 3D-μPAD is used for the simultaneous colorimetric detection of heavy metal ions in a concentration range of 0.1-2000 ppm, including nickel (Ni2+), copper (Cu2+), mercury (Hg2+), and radioactive isotope cesium-137 ions (Cs+). We confirm that qualitative analysis and image-based quantitative analysis with high reliability are possible through rapid color changes within 3 min. The limits of detection (LOD) for 0.55 ppm of Ni2+, 5.05 ppm of Cu2+, 0.188 ppm of Hg2+, and 0.016 ppm of Cs+ are observed, respectively. In addition, we confirm that the analysis is highly reliable in a wide range of ion concentrations with CV values below 3% for Ni2+ (0.56%), Cu2+ (0.45%), Hg2+ (1.35%), and Cs+ (2.18%). This method could be a promising technique to develop a 3D-μPAD with various applications as a primary early analysis device in the environmental and biological industries.

Highly Sensitive Photothermal Microfluidic Thread-Based Duplex Immunosensor for Point-of-Care Monitoring

Herein, we successfully developed a thread-based analytical device (μTAD) for simultaneous immunosensing of two biomolecules with attomolar sensitivity by using a photothermal effect. A photothermal effect exploits a strong light-to-heat energy conversion of plasmonic metallic nanoparticles at localized surface plasmon resonance. The key innovation is to utilize the cotton thread to realize this sensor and the use of chitosan modification for enhancing the microfluidic properties, for improving the efficiency of photothermal conversion, and for sensor stability. The developed μTAD sensor consists of (i) a sample zone, (ii) a conjugation zone coated with gold nanoparticles bound with an antibody (AuNPs-Ab2), and (iii) a test zone immobilized with a capture antibody (anti-Ab1). The prepared μTAD is assembled in a custom three-dimensional (3D) printed device which holds the laser for illumination and the thermometer for readout. The 3D-printed supportive device enhances signal response by focusing light and localizing the heat generated. For proof of concept, simultaneous sensing of two key stress and inflammation biomarkers, namely, cortisol and interleukin-6 (IL-6), are monitored using this technique. Under optimization, this device exhibited a detection linear range of 2.0-14.0 ag/mL (R2 = 0.9988) and 30.0-360.0 fg/mL (R2 = 0.9942) with a detection limit (LOD) of 1.40 ag/mL (∼3.86 amol/L) and 20.0 fg/mL (∼950.0 amol/L) for cortisol and IL-6, respectively. Furthermore, the analysis of both biomolecules in human samples indicated recoveries in the range of 98.8%-102.88% with the highest relative standard deviation being 3.49%, offering great accuracy and precision. These results are the highest reported sensitivity for these analytes using an immunoassay method. Our PT-μTAD strategy is therefore a promising approach for detecting biomolecules in resource-limited point-of-care settings.

Advances in paper based isothermal nucleic acid amplification tests for water-related infectious diseases

Water-associated or water-related infectious disease outbreaks are caused by pathogens such as bacteria, viruses, and protozoa, which can be transmitted through contaminated water sources, poor sanitation practices, or insect vectors. Low- and middle-income countries bear the major burden of these infections due to inadequate hygiene and subpar laboratory facilities, making it challenging to monitor and detect infections in a timely manner. However, even developed countries are not immune to these diseases, as inadequate wastewater management and contaminated drinking water supplies can also contribute to disease outbreaks. Nucleic acid amplification tests have proven to be effective for early disease intervention and surveillance of both new and existing diseases. In recent years, paper-based diagnostic devices have made significant progress and become an essential tool in detecting and managing water-associated diseases. In this review, we highlight the importance of paper and its variants as a diagnostic tool and discuss the properties, design modifications, and various paper-based device formats developed and used for detecting water-associated pathogens.

Alternative platform for COVID-19 diagnosis based on AuNP-modified lab-on-paper

COVID-19 has caused global health problems, and so rapid diagnosis is crucial to slow spread of the disease. Herein, a novel lab-on-paper screening method for SARS-CoV-2 Omicron BA.2 variant was developed using a gold nanoparticle-based colorimetric biosensor along with sensitive detection of SARS-CoV-2 antigen using laser desorption ionization-mass spectrometry (LDI-MS). As a result of antigen-antibody interaction, in the presence of SARS-CoV-2 antigen the gold nanoparticles undergo aggregation and change color from red to light purple, allowing for rapid determination of SARS-CoV-2 antigen with the naked eye. Furthermore, the lab-on-paper method can be directly applied as a substrate for sensitive quantitation of SARS-CoV-2 antigen in saliva using LDI-MS without the use of a conventional organic matrix and sample preparation. LDI-MS offers early diagnosis with high sensitivity, rapidity without sample preparation and lower cost per test compared with reverse transcriptase-PCR, which is crucial for preventing mortality in patients with underlying conditions. This method showed linearity over 0.01-1 μg mL^-1 covering the cut-off value of 0.048 μg mL^-1 for COVID-19 detection in human saliva. Moreover, a colorimetric sensor for urea was also fabricated in-parallel, for prediction of COVID-19 severity in patients with chronic kidney disease. The color change upon increasing urea concentration directly reflected kidney damage, which is related to increasing risk of mortality among patients with COVID-19. Hence, this platform might be a potential device for non-invasive diagnosis of SARS-CoV-2 Omicron BA.2 variant, which is the variant of most concern because it is transmitted more rapidly than the original SARS-CoV-2 virus and the Delta variant.

A Paper-Based Fluorescent Sensor for Rapid Early Screening of Oral Squamous Cell Carcinoma

Various types of sensors play an irreplaceable role in the detection of biomarkers, but their high cost and complicated operation make it difficult to benefit ordinary people. Herein, we develop a low-cost, double-layered, paper-based fluorescent sensor (CP/HQ) structurally consisting of the upper reaction layer loaded with two oxidases (lactate oxidase and choline oxidase) and the bottom fluorescent layer that physically associates with the porphine-grafted composite fluorescent polymer colloids (PF-PDMTP/HQ). Based on the dramatic and rapid fluorescence decrease of porphine induced by the oxidation between saliva and oxidases and subsequent fluorescence resonance energy transfer from oxidized hydroquinone, the resultant fluorescent paper sensor enables us to achieve visual detection of OSCC, which was further recognized by smartphone scanning as the grayscale variation. It was found that the linear sensing range of grayscale value are 10-200 μM for lactic acid and 10-100 μM for choline, with LODs of 5.7 and 8.9 μM, respectively. More importantly, the sensor can achieve a powerful detection capability comparable to that of high-performance liquid chromatography (HPLC) in clinical settings with simple operation, demonstrating its great application potential. Our proposed sensor not only improves the accuracy of OSCC diagnosis but also provides a valuable attempt for the device modification of polymer-sensing systems and the development of non-invasive and easy-to-operate disease screening methods.

Conductive Nanofibers-Enhanced Microfluidic Device for the Efficient Capture and Electrical Stimulation-Triggered Rapid Release of Circulating Tumor Cells

The effective detection and release of circulating tumor cells (CTCs) are of great significance for cancer diagnosis and monitoring. The microfluidic technique has proved to be a promising method for CTCs isolation and subsequent analysis. However, complex micro-geometries or nanostructures were often constructed and functionalized to improve the capture efficiency, which limited the scale-up for high-throughput production and larger-scale clinical applications. Thus, we designed a simple conductive nanofiber chip (CNF-Chip)-embedded microfluidic device with a herringbone microchannel to achieve the efficient and specific capture and electrical stimulation-triggered rapid release of CTCs. Here, the most used epithelial cell adhesion molecule (EpCAM) was selected as the representative biomarker, and the EpCAM-positive cancer cells were mainly studied. Under the effects of the nanointerface formed by the nanofibers with a rough surface and the herringbone-based high-throughput microfluidic mixing, the local topographic interaction between target cells and nanofibrous substrate in the microfluidic was synergistically enhanced, and the capture efficiency for CTCs was further improved (more than 85%). After capture, the sensitive and rapid release of CTCs (release efficiency above 97%) could be conveniently achieved through the cleavage of the gold-sulfur bond by applying a low voltage (-1.2 V). The device was successfully used for the effective isolation of CTCs in clinical blood samples from cancer patients, indicating the great potential of this CNF-Chip-embedded microfluidic device in clinical applications.

A single thiolated-phage displayed nanobody-based biosensor for label-free detection of foodborne pathogen

Rapid and sensitive detection of bacterial pathogens present in food and environmental samples is of crucial importance to ensure human health and safety. Here, we present a one-step label-free colorimetric strategy based on M13 bacteriophage-displayed nanobody (phage-Nb) derived from camelid heavy-chain antibodies specific to Vibrio parahaemolyticus (V. parahaemolyticus). The thiolation of phage-Nb (Phage-Nb-SH) on pVIII shell proteins induces the aggregation of gold nanoparticles (AuNPs), whereas the specific interaction between nanobody and bacteria prevents the aggregation of AuNPs, resulting in visible color change due to alteration of surface plasmon resonance properties. Based on this phenomenon, a simple and sensitive colorimetric immunosensor for V. parahaemolyticus was developed. The assay can be accomplished within 100 min, and exhibits a visual detection limit of 10⁴ cfu/mL and a quantitative detection limit of 10³ cfu/mL, with no cross-reactivity towards other bacterial species. This strategy takes full advantages of both the high specificity of phage-Nbs and the optical properties of AuNPs, enabling simple and rapid detection of bacterial pathogens.

Translational Nanomedicines Across Human Reproductive Organs Modeling on Microfluidic Chips: State-of-the-Art and Future Prospects

Forecasting the consequence of nanoparticles (NPs) and therapeutically significant molecules before materializing for human clinical trials is a mainstay for drug delivery and screening processes. One of the noteworthy obstacles that has prevented the clinical translation of NP-based drug delivery systems and novel drugs is the lack of effective preclinical platforms. As a revolutionary technology, the organ-on-a-chip (OOC), a coalition of microfluidics and tissue engineering, has surfaced as an alternative to orthodox screening platforms. OOC technology recapitulates the structural and physiological features of human organs along with intercommunications between tissues on a chip. The current review discusses the concept of microfluidics and confers cutting-edge fabrication processes for chip designing. We also outlined the advantages of microfluidics in analyzing NPs in terms of characterization, transport, and degradation in biological systems. The review further elaborates the scope and research on translational nanomedicines in human reproductive organs (testis, placenta, uterus, and menstrual cycle) by taking the advantages offered by microfluidics and shedding light on their potential future implications. Finally, we accentuate the existing challenges for clinical translation and scale-up dynamics for microfluidics chips and emphasize its future perspectives.

Rapid identification and quantitation of single plant seed allergen using paper-based microfluidics

Nucleic acid amplification is a sensitive and powerful tool for allergen detection. However, it is limited due to the relatively cumbersome methods required to extract nucleic acids from single plant seed allergen (e.g. peanut and soybean). In view of this, an approach of extracting nucleic acid with untreated glass-fiber paper (paper-based microfluidics) was applied for nucleic acid capture and purification from plant seed allergen and commercial products. After cut by hollow cylindrical cutter, a certain size the paper chip it used to absorb DNA. And this paper-based microfluidics with DNA was directly applied for amplification by loop-mediated isothermal amplification (LAMP). To evaluate the adsorption performance of paper chip to DNA, CTAB and SDS method were used as comparisons. From amplification results, the established technique has good specificity, high repeatability (C.V. values are 4.41% and 6.17% for peanut and soybean) and favorable sensitivity (7.39 ng/μL or peanut and 6.6 ng/μL for soybean), and successfully used for commercial products (2 kinds of candy and 2 kinds of cakes containing peanut, and 2 kinds of drinks, candy and 2 kinds of biscuits containing soybean). This speed and flexible detection method makes it suit for applications in point-of-care (POC) detection at different scenario, such as custom house and import port.

Microfluidic platforms integrated with nano-sensors for point-of-care bioanalysis

Microfluidic technology provides a portable, cost-effective, and versatile tool for point-of-care (POC) bioanalysis because of its associated advantages such as fast analysis, low volumes of reagent consumption, and high portability. Along with microfluidics, the application of nanomaterials in biosensing has attracted lots of attention due to their unique physical and chemical properties for enhanced signal modulation such as signal amplification and signal transduction for POC bioanalysis. Hence, an enormous number of microfluidic devices integrated with nano-sensors have been developed for POC bioanalysis targeting low-resource settings. Herein, we review recent advances in POC bioanalysis on nano-sensor-based microfluidic platforms. We first briefly summarized the different types of cost-effective microfluidic platforms, followed by a concise introduction to nanomaterial-based biosensors. Then, we highlighted the application of microfluidic platforms integrated with nano-sensors for POC bioanalysis. Finally, we discussed the current limitations and perspective trends of the nano-sensor-based microfluidic platforms for POC bioanalysis.

Nanozyme-based colorimetric biosensor with a systemic quantification algorithm for noninvasive glucose monitoring

Diabetes mellitus accompanies an abnormally high glucose level in the bloodstream. Early diagnosis and proper glycemic management of blood glucose are essential to prevent further progression and complications. Biosensor-based colorimetric detection has progressed and shown potential in portable and inexpensive daily assessment of glucose levels because of its simplicity, low-cost, and convenient operation without sophisticated instrumentation. Colorimetric glucose biosensors commonly use natural enzymes that recognize glucose and chromophores that detect enzymatic reaction products. However, many natural enzymes have inherent defects, limiting their extensive application. Recently, nanozyme-based colorimetric detection has drawn attention due to its merits including high sensitivity, stability under strict reaction conditions, flexible structural design with low-cost materials, and adjustable catalytic activities. This review discusses various nanozyme materials, colorimetric analytic methods and mechanisms, recent machine learning based analytic methods, quantification systems, applications and future directions for monitoring and managing diabetes.

Nanomaterials-assisted metabolic analysis toward in vitro diagnostics

In vitro diagnostics (IVD) has played an indispensable role in healthcare system by providing necessary information to indicate disease condition and guide therapeutic decision. Metabolic analysis can be the primary choice to facilitate the IVD since it characterizes the downstream metabolites and offers real-time feedback of the human body. Nanomaterials with well-designed composition and nanostructure have been developed for the construction of high-performance detection platforms toward metabolic analysis. Herein, we summarize the recent progress of nanomaterials-assisted metabolic analysis and the related applications in IVD. We first introduce the important role that nanomaterials play in metabolic analysis when coupled with different detection platforms, including electrochemical sensors, optical spectrometry, and mass spectrometry. We further highlight the nanomaterials-assisted metabolic analysis toward IVD applications, from the perspectives of both the targeted biomarker quantitation and untargeted fingerprint extraction. This review provides fundamental insights into the function of nanomaterials in metabolic analysis, thus facilitating the design of next-generation diagnostic devices in clinical practice.

Gold Tablets: Gold Nanoparticles Encapsulated into Dextran Tablets and Their pH-Responsive Behavior as an Easy-to-Use Platform for Multipurpose Applications

Many applications using gold nanoparticles (AuNPs) require (i) their functionalization with a biopolymer to increase their stability and (ii) their transformation into an easy-to-handle material, which provide them with specific properties. In this research, a portable tablet platform is presented based on dextran-encapsulated gold nanoparticles (AuNPs-dTab) by a ligand exchange reaction between citrate-capped gold nanoparticles (AuNPs-Cit) and dextran. These newly fabricated tablets were characterized utilizing ultraviolet-visible spectroscopy (UV-vis), Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR), transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray diffraction spectroscopy (XRD), differential scanning calorimetry (DSC), and atomic force microscopy (AFM) techniques. The results showed that dextran-capped gold nanoparticles in a tablet platform (AuNPs-dTab) were well-dispersed and highly stable for at least a year at room temperature. In addition to particle and surface characterization of AuNPs-dTab, the tablet morphology in terms of thickness, diameter, density, and opacity was also measured using 6 and 10% dextran with 2, 4 and 8 nM AuNPs-Cit. We further investigated the pH-responsive behavior of AuNPs-dTab in the presence and absence of sodium chloride. Results showed that neutral and alkaline environments were suitable to render AuNPs dispersed in a tablet, while an acidic condition controls the aggregation rate of AuNPs as confirmed by concentration-dependent aggregation phenomena. Besides the easy fabrication, these tablets were portable and low-cost (approx. 1.22 CAD per 100 tablets of a 100 μL solution of dextran-capped gold nanoparticles (AuNPs-dSol)). The biocompatible nature of dextran along with the acidic medium trigger nature of AuNPs makes our proposed tablet a potential candidate for cancer therapy due to the acidic surrounding of tumor tissues as compared to normal cells. Also, our proposed tablet approach paves the way for the fabrication of portable and easy-to-use optical sensors based on the AuNPs embedded in a natural polymeric architecture that would serve as a colorimetric recognition indicator for detecting analytes of interest.

Waterproof Cellulose-Based Substrates for In-Drop Plasmonic Colorimetric Sensing of Volatiles: Application to Acid-Labile Sulfide Determination in Waters

The present work reports on the assessment of widely available waterproof cellulose-based substrates for the development of sensitive in-drop plasmonic sensing approaches. The applicability of three inexpensive substrates, namely, Whatman 1PS, polyethylene-coated filter paper, and tracing paper, as holders for microvolumes of colloidal solutions was evaluated. Waterproof cellulose-based substrates demonstrated to be highly convenient platforms for analytical purposes, as they enabled in situ generation of volatiles and syringeless drop exposure unlike conventional single-drop microextraction approaches and can behave as sample compartments for smartphone-based colorimetric sensing in an integrated way. Remarkably, large drop volumes (≥20 μL) of colloidal solutions can be employed for enrichment processes when using Whatman 1PS as holder. In addition, the stability and potential applicability of spherical, rod-shaped, and core-shell metallic NPs onto waterproof cellulose-based substrates was evaluated. In particular, Au@AgNPs showed potential for the colorimetric detection of in situ generated H2S, I2, and Br2, whereas AuNRs hold promise for I2, Br2, and Hg0 colorimetric sensing. As a proof of concept, a smartphone-based colorimetric assay for determination of acid-labile sulfide in environmental water samples was developed with the proposed approach taking advantage of the ability of Au@AgNPs for H2S sensing. The assay showed a limit of detection of 0.46 μM and a repeatability of 4.4% (N = 8), yielding satisfactory recoveries (91-107%) when applied to the analysis of environmental waters.

Gold nanoparticles spontaneously grown on cellulose nanofibrils as a reusable nanozyme for colorimetric detection of cholesterol in human serum

Recently, gold nanoparticles (AuNPs) are extensively used as peroxidase mimics. However, low catalytic activity, high synthesis cost, substrate-induced aggregation in reaction medium and difficulty in recovery and reuse still remain as major challenges. Here, a novel, simple, spontaneous, and reagent-less in-situ method for the production of AuNPs using dialdehyde cellulose nanofibrils (DACNF) is proposed. AuNPs synthesis time and size were greatly influenced by aldehyde content and the optimal aldehyde content for ultra-small AuNPs (≈10 nm) was 2.1 mM/g. AuNPs@DACNFs exhibited broad-spectrum peroxidase activity and steady-state kinetics revealed their better kinetic parameters (low K_m and high V_max) over horseradish peroxidase (HRP). AuNPs@DACNFs was further converted into paper strip, which served as a biosensor for H₂O₂ and cholesterol detection. The proposed method exhibited wide linear response in the range of 10-90 μM and 0.05-0.45 mM, and detection limit of 0.39 μM and 1.9 μM for H₂O₂ and cholesterol, respectively. Great shelf life and reusability were evident by FE-SEM and ICP-OES analysis. The smartphone application "Color Grab" was used to enable the portable onsite detection. The results of cholesterol detection in human serum samples were in agreement with clinically observed values, suggesting the great potential of the probe in disease diagnosis.

Nanomaterial-assisted microfluidics for multiplex assays

Simultaneous detection of different biomarkers from a single specimen in a single test, allowing more rapid, efficient, and low-cost analysis, is of great significance for accurate diagnosis of disease and efficient monitoring of therapy. Recently, developments in microfabrication and nanotechnology have advanced the integration of nanomaterials in microfluidic devices toward multiplex assays of biomarkers, combining both the advantages of microfluidics and the unique properties of nanomaterials. In this review, we focus on the state of the art in multiplexed detection of biomarkers based on nanomaterial-assisted microfluidics. Following an overview of the typical microfluidic analytical techniques and the most commonly used nanomaterials for biochemistry analysis, we highlight in detail the nanomaterial-assisted microfluidic strategies for different biomarkers. These highly integrated platforms with minimum sample consumption, high sensitivity and specificity, low detection limit, enhanced signals, and reduced detection time have been extensively applied in various domains and show great potential in future point-of-care testing and clinical diagnostics.

Plasmonic nanosensors for point-of-care biomarker detection

Advancement of materials along with their fascinating properties play increasingly important role in facilitating the rapid progress in medicine. An excellent example is the recent development of biosensors based on nanomaterials that induce surface plasmon effect for screening biomarkers of various diseases ranging from cancer to Covid-19. The recent global pandemic re-confirmed the trend of real-time diagnosis in public health to be in point-of-care (POC) settings that can screen interested biomarkers at home, or literally anywhere else, at any time. Plasmonic biosensors, thanks to its versatile designs and extraordinary sensitivities, can be scaled into small and portable devices for POC diagnostic tools. In the meantime, efforts are being made to speed up, simplify and lower the cost of the signal readout process including converting the conventional heavy laboratory instruments into lightweight handheld devices. This article reviews the recent progress on the design of plasmonic nanomaterial-based biosensors for biomarker detection with a perspective of POC applications. After briefly introducing the plasmonic detection working mechanisms and devices, the selected highlights in the field focusing on the technology's design including nanomaterials development, structure assembly, and target applications are presented and analyzed. In parallel, discussions on the sensor's current or potential applicability in POC diagnosis are provided. Finally, challenges and opportunities in plasmonic biosensor for biomarker detection, such as the current Covid-19 pandemic and its testing using plasmonic biosensor and incorporation of machine learning algorithms are discussed.

A Colorimetric Immunoassay Based on g-C3N4@Fe3O4 Nanocomposite for Detection of Carcinoembryonic Antigen

We proposed a colorimetric immunosensor based on g-C₃N₄@Fe₃O₄ nanocomposite-mediated transformation strategy for sensitive detection of carcinoembryonic antigen (CEA). The g-C₃N₄@Fe₃O₄ nanocomposite was synthesized and characterized by the scanning electron microscope (SEM), energy dispersive X-ray spectra (EDX), X-ray powder diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). Fe³⁺ derived from g-C₃N₄@Fe₃O₄ nanocomposite could combine with sodium salicylate to form purple complex products. Based on this color development, the sandwich colorimetric immunoassay was built by utilizing g-C₃N₄@Fe₃O₄ nanocomposite as nanolabels on the microplate. With the increasement of CEA concentration, the purple color showed a gradient change. Under optimal conditions, the linearity range is 0.001-50 ng/mL with the detection limit of 0.35 pg/mL for CEA. More importantly, the colorimetric immunoassay has good selectivity, specificity, repeatability, and stability.

Recent Advances and Applications in Paper-Based Devices for Point-of-Care Testing

Point-of-care testing (POCT), as a portable and user-friendly technology, can obtain accurate test results immediately at the sampling point. Nowadays, microfluidic paper-based analysis devices (μPads) have attracted the eye of the public and accelerated the development of POCT. A variety of detection methods are combined with μPads to realize precise, rapid and sensitive POCT. This article mainly introduced the development of electrochemistry and optical detection methods on μPads for POCT and their applications on disease analysis, environmental monitoring and food control in the past 5 years. Finally, the challenges and future development prospects of μPads for POCT were discussed.

Advances in multiplex electrical and optical detection of biomarkers using microfluidic devices

Microfluidic devices can provide a versatile, cost-effective platform for disease diagnostics and risk assessment by quantifying biomarkers. In particular, simultaneous testing of several biomarkers can be powerful. Here, we critically review work from the previous 4 years up to February 2021 on developing microfluidic devices for multiplexed detection of biomarkers from samples. We focus on two principal approaches: electrical and optical detection methods that can distinguish and quantify biomarkers. Both electrical and spectroscopic multiplexed detection strategies are being employed to reach limits of detection below clinical sample levels. Some of the most promising strategies for point-of-care assays involve inexpensive materials such as paper-based microfluidic devices, or portable and accessible detectors such as smartphones. This review does not comprehensively cover all multiplexed microfluidic biomarker studies, but rather provides a critical evaluation of key work and suggests promising prospects for future advancement in this field. Electrical and optical multiplexing are powerful approaches for microfluidic biomarker analysis.

A Smartphone Optical Device for Point-of-Care Testing of Glucose and Cholesterol Using Ag NPs/UiO-66-NH2-Based Ratiometric Fluorescent Probe

Point-of-care testing (POCT) with the advantages of simplicity, rapidity, portability, and low-cost is of great importance to improve healthcare, especially in resource-limited settings and home healthcare settings. Moreover, it is a great challenge to quantitative POCT of multiplexed biomarkers within a single accessible assay but provides enhanced diagnostic accuracy and improved diagnostic efficiency. Herein, a smartphone optical device has been designed for POCT of glucose and cholesterol in metabolic syndrome patients using a ratiometric fluorescent sensor. The sensing system of Ag NPs/UiO-66-NH₂ and o-phenylenediamine presents a dual-emission response to H₂O₂ (the main product of glucose and cholesterol catalyzed by glucose oxidase and cholesterol oxidase) on account of the inner filter effect, resulting in an increase in the response of the fluorescence intensity ratio (F_555 nm/F_425 nm) accompanied by a distinguishable color transition from blue to yellow green. After compositing probes with a flexible substrate, the obtained test strip can be integrated with a smartphone-based portable platform to read RGB values for accurate testing of glucose and cholesterol with both detection limits of 10 μmol L^-1, which are hundreds of times lower than their concentrations in human serum. With the advantages of low-cost, ease of operation, and broad adaptability, this smartphone optical device holds great potential for portable detection of numerous targets in personalized healthcare and clinical diagnosis.

Paper-Based Biosensors for COVID-19: A Review of Innovative Tools for Controlling the Pandemic

The appearance and quick spread of the new severe acute respiratory syndrome coronavirus disease, COVID-19, brought major societal challenges. Importantly, suitable medical diagnosis procedures and smooth clinical management of the disease are an emergent need, which must be anchored on novel diagnostic methods and devices. Novel molecular diagnostic tools relying on nucleic acid amplification testing have emerged globally and are the current gold standard in COVID-19 diagnosis. However, the need for widespread testing methodologies for fast, effective testing in multiple epidemiological scenarios remains a crucial step in the fight against the COVID-19 pandemic. Biosensors have previously shown the potential for cost-effective and accessible diagnostics, finding applications in settings where conventional, laboratorial techniques may not be readily employed. Paper- and cellulose-based biosensors can be particularly relevant in pandemic times, for the renewability, possibility of mass production with sustainable methodologies, and safe environmental disposal. In this review, paper-based devices and platforms targeting SARS-CoV-2 are showcased and discussed, as a means to achieve quick and low-cost PoC diagnosis, including detection methodologies for viral genomic material, viral antigen detection, and serological antibody testing. Devices targeting inflammatory markers relevant for COVID-19 are also discussed, as fast, reliable bedside diagnostic tools for patient treatment and follow-up.

Photothermal immunoassay for carcinoembryonic antigen based on the inhibition of cysteine-induced aggregation of gold nanoparticles by copper ion using a common thermometer as readout

The dispersed gold nanoparticles (AuNPs) have weak photothermal effect in near-infrared (NIR) region. After the addition of cysteine, the AuNPs are aggregated due to the electrostatic interaction and then exhibited strong photothermal effect. At present of copper ion (Cu²⁺), the cysteine was catalytically oxidized into cystine, leading to the inhibition of the aggregation of AuNPs and the photothermal effect decreased. Based on this, a simple photothermal assay can be developed for Cu²⁺ detection using a common thermometer as readout. The change of the temperature (ΔT) of the system has a linear relationship with Cu²⁺ in the range of 10-300 nM with a detection limit of 7.4 nM (S/N = 3). Furthermore, through labeling the detection antibody in immunoassay with CuO nanoparticles as the source of Cu²⁺, a convenient photothermal immunoassay can be developed. Carcinoembryonic antigen (CEA), an important biomarker for cancer screening, was chosen as the model target because the rise of CEA level is widely present in cancer blood serum. Under the optimized conditions, ΔT has a linear relationship with CEA concentration in the range of 3.0-48.0 ng/mL. The detection limit is 1.3 ng/mL. The proposed method had been applied to detect CEA in serum samples with good agreement with the reference method used in hospital.

Enhanced Catalytic Activity Induced by the Nanostructuring Effect in Pd Decoration onto Doped Ceria Enabling an Origami Paper Analytical Device for High Performance of Amyloid-β Bioassay

In this work, we fabricated a novel origami paper-based analytical device (oPAD) assisted by the nanostructuring effect of in situ Pd decoration of Cu/Co-doped CeO₂ (CuCo-CeO₂-Pd) nanospheres, functionalized with their strongly enhanced electrocatalytic properties to realize an electrochemical and visual signal readout system in oPAD, for highly sensitive detection of amyloid-β (Aβ). The CuCo-CeO₂-Pd nanospheres were introduced as an enhanced "signal transducer layer" on account of the electron transfer acceleration caused by catalyzing glucose to produce H₂O₂ for differential pulse voltammetry signal readout and further 3,3'5,5'-tetramethylbenzidine (TMB) oxidation for colorimetric analysis. Meanwhile, for achieving superior performance of the proposed oPAD, in situ growth of urchin-like gold nanoparticles (Au NPs) onto cellulose fibers was adopted to improve "the recognition layer" in favor of immobilizing antibodies for targeting Aβ through specific antigen-antibody interactions. Combined with the delicate design of oPAD, exhibiting actuation of the conversion procedure between hydrophobicity and hydrophilicity on paper tabs in the assay process, the oPAD successfully enabled sensitive diagnosis of Aβ in a linear range from 1.0 pM to 100 nM with a limit of detection of 0.05 pM (S/N = 3) for electrochemical detection, providing a reliable strategy for quantifying the Aβ protein in clinical applications.

Bi2S3@MoS2 Nanoflowers on Cellulose Fibers Combined with Octahedral CeO2 for Dual-Mode Microfluidic Paper-Based MiRNA-141 Sensors

An effective dual-mode microfluidic paper-based analysis device (μPAD) was proposed via Bi₂S₃@MoS₂ nanoflowers combined with octahedral CeO₂ for ultrasensitive miRNA-141 bioassay. To obtain the amplified electrochemical signal, Bi₂S₃@MoS₂ nanoflowers were first in situ grown onto the surface of cellulose fibers to promote the reduction of H₂O₂. The prism-anchored octahedral CeO₂ nanoparticles with a great catalytic function on the reduction of H₂O₂ were linked up to the functionalized cellulose fibers through the hybridization chain reaction to further enhance the electrochemical signal. By means of the catalysis effect of Bi₂S₃@MoS₂ nanoflowers and octahedral CeO₂ nanoparticles, the obtained signal was amplified, thereby achieving ultrasensitive electrochemical detection of the target. With the help of duplex specific nuclease, the octahedral CeO₂ could be released from the electrochemical detection area and flow to the color channel through capillary action, which could initiate the oxidation reaction of 3,3',5,5'-tetramethylbenzidine in the existence of H₂O₂ to generate a blue visual band, avoiding the error of distinguishing color depth caused by the naked eye and thus improving the accuracy of the visual method. Under the optimal conditions, satisfactory prediction and accurate detection performance were achieved in the range of 10 fM-1 nM and 0.5 fM-1 nM, respectively, by measuring the length of the blue product and the electrochemical signal intensity. The electrochemical/visual detection limits of the proposed μPAD for miRNA-141 were as low as 0.12 and 2.65 fM (S/N = 3). This work provides great potential for the construction of low-cost and high-performance dual-mode biosensors for the detection of biomarkers.