A paper-based optical probe for chromium by using gold nanoparticles modified with 2,2′-thiodiacetic acid and smartphone camera readout

Advances in nanoparticle-enhanced paper sensor for detecting toxic metals in water

Toxic metals in water pose a serious threat to public health and the environment, especially in regions with limited access to advanced laboratory infrastructure. Traditional methods for detecting toxic metals, such as atomic absorption spectrometry (AAS) and inductively coupled plasma mass spectrometry (ICP-MS), are accurate but expensive, complex, and unsuitable for on-site use. In contrast, paper-based analytical devices (PADs) offer a low-cost, and user-friendly alternative, especially in resource-limited areas. Recent advancements have significantly improved PAD performance, especially through the integration of nanoparticles and the use of enhanced colorimetric and electrochemical detection methods. These improvements have enabled faster, more sensitive detection while maintaining simplicity and field readiness. This review explores key advancements in PAD technology from 2015 to 2025 including advances in sensitivity, nanoparticle functionalization, and smartphone-based readouts. Unlike previous reviews, this study presents a comparative analysis of PAD detection mechanisms, evaluates commercialization and regulatory challenges, and explores emerging trends such as smartphone integration and microextraction techniques. By addressing these aspects, this review highlights key advancements and optimization strategies to enhance the stability, selectivity, and practical implementation of PADs for water quality monitoring.

An Ultra-Sensitive Colorimetric Sensing Platform for Simultaneous Detection of Moxifloxacin/Ciprofloxacin and Cr(III) Ions Based on Ammonium Thioglycolate Functionalized Gold Nanoparticles

Water pollution by antibiotics and heavy metals threatens the ecological environment and human health globally, yet there is no rapid method to detect multiple antibiotics and metal ions simultaneously. A simple, fast, and ultra-sensitive colorimetric chemosensor for the simultaneous detection of moxifloxacin (MOX), ciprofloxacin (CIP), and Cr(III) based on the aggregation of ammonium thioglycolate (ATG)-functionalized gold nanoparticles (ATG-AuNPs) was developed. Following the addition of MOX, CIP, and Cr(III), a color change in the solution was observed from wine-red to blue-grey. The UV-Vis signal of the ATG-AuNPs system blended with MOX, CIP, and Cr(III) in the range of 0~200 µM, 0~100 µM, and 0~5 µM was assessed and measured with detection limits (LODs) of 1.57 µM, 1.30 µM, and 57.1 nM calculated by 3σ/S, respectively. Therefore, this system has the potential to act as an effective colorimetric chemosensor for simultaneously detecting MOX, CIP, and Cr(III) in complex environmental systems.

Chromium Detection in Water Using Optical Methods: A Study of Reagent and Reagentless Approaches

Water contaminated with chromium (Cr) poses significant risks to public health and the environment, necessitating reliable detection techniques. This review study uniquely provides a comprehensive analysis of optical methods for detecting Cr pollution in water, focusing on both reagent-based and reagentless approaches, as well as various sensing platforms. Unlike existing reviews that primarily focus on electrochemical and colorimetric/fluorimetric methods, this work highlights the untapped potential of optical technologies, such as colorimetry, SPR, UV-Vis spectroscopy, and more, in detecting distinct Cr species, including reagent and reagentless based approaches. The findings demonstrate the high sensitivity and specificity of optical methods. Reagent-based approaches offer exceptional sensitivity but involve complex preparation and potential secondary contamination. In contrast, reagentless methods, while requiring sophisticated calibration, are more environmentally friendly and simpler to implement. Future directions emphasize the development of portable, cost-effective optical devices, improved Cr species differentiation, and integration with real-time data processing and remote sensing for enhanced field monitoring. This study informs researchers and policymakers about the latest advancements in optical detection techniques and their potential to enhance water quality monitoring.

Chitosan-stabilized gold nanoparticles decorated with a thiodiacetic acid nanoprobe for selective detection of arsenic(iii) in rice and water samples

A sensitive and selective method for the detection of arsenic(iii) (As3+) based on chitosan-stabilized gold nanoparticles (CS/AuNPs) decorated with a 2,2'-thiodiacetic acid (TDA) nanoprobe was developed and used to detect and indicate the contamination of rice, drinking water and environmental water samples. AuNPs were reduced and stabilized with CS and subsequently functionalized with TDA. As3+ interacted with the carboxylate group of TDA to form an As-TDA complex, inducing the aggregation of CS/AuNPs@TDA. The aggregation of CS/AuNPs@TDA was accompanied with a change in color from red to bluish purple and a shift in surface plasmon resonance wavelength from 525 nm to 645 nm. The response for the detection of As3+ was linear at concentrations from 10 to 1000 μg L-1 with a limit of detection of 6.1 μg L-1. The method exhibited selectivity toward As3+ among various cations (As5+, Cu2+, Fe3+, Fe2+, Hg2+, Al3+, Cr3+, Cd2+, Co2+, Ni2+, Pb2+ and Zn2+) and anions (Br-, Cl-, F-, SO4 2-, NO3 - and PO4 2-). The CS/AuNPs@TDA nanoprobe was applied to detect As3+ in rice, drinking water and environmental water samples. The results were consistent with those obtained via inductively coupled plasma-optical emission spectrometry (ICP-OES). Satisfactory recoveries ranging from 88.22% to 105.74% (RSDs of 0.25-2.99%) were obtained from spiked samples.

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.

Paper-based optical sensors paired with smartphones for biomedical analysis

The traditional analytical methods used for biomedical analysis are expensive and not easy to handle and require sophisticated instruments, thus their application is limited in resource-limited settings. Due to their portability, low cost, and ability to be applied to different analytical techniques, paper-based analytical devices are becoming valuable tools for biomedical analysis. The integration of smartphones into analytical devices has provided the ability to build portable, cost-effective, straightforward analytical devices for biomedical analysis and mobile health. The key aim of this review is to emphasize the recent applications of PADs combined with a smartphone for the optical analysis of biomedical species. We started this review by highlighting the type of papers and their modifications with different materials to prepare the PADs. After that, this review presents various detection methods including colorimetry, fluorescence, and luminescence where the smartphone is used for read-out. In the end, we provided the recent applications of the analysis of different biomedical compounds such as cancer and cardiovascular biomarkers, metal ions, glucose, viruses, etc. We believe that the present review will attract a wide scientific community in the areas of analytical chemistry, sensors, and clinical testing.

PMMA/paper hybrid microfluidic chip for simultaneous determination of arginine and valine in human plasma

The simultaneous determination is reported of arginine (Arg) and valine (Val) amino acids in plasma using flower-shaped μPADs and PMMA/paper hybrid microfluidic chip based on AuNPs capped with R-thiazolidine-4-carboxylic acid (THP). In this article, the evaluation procedure is based on the smartphone colorimetric detection mechanism that results from the aggregation of the THP-AuNPs with the addition of amino acids and visual color change from red to blue. Arg and Val were selectively determined with good reproducibility and an acceptable linearity range. The flower-shaped (μPADs) provides many advantages, including low cost, reasonable sensitivity, simple and fast performance, simultaneous detection, disposable use, and high sample throughput compared with conventional colorimetric method using cuvette cells. The ratios between the absorbance wavelength at (A₆₅₀/A₅₂₅) and (A₆₈₅/A₅₂₅) are linearly proportional to the concentration of Arg and Val. Under the optimum conditions, the calibration range in aqueous solutions is 0.0068-100.0 and 0.0056-75.0 µM with a limit of detection of 2.25 and 1.86 nM for Arg and Val at pH 7.0, respectively. In the case of μPADs, the calibration curves for Arg and Val showed good linearity in the concentration range 0.01-75.0 µM. The detection limits for the analytes were 3.51 nM and 3.44 nM for Arg and Val, respectively. In addition, a PMMA/paper hybrid microfluidic chip was successfully employed to determine Arg and Val in plasma samples with a relative error below 5%.

Smartphone-based microfluidic chip modified using pyrrolidine-1-dithiocarboxylic acid for simultaneous colorimetric determination of Cr3+ and Al3+ ions

A portable µ-chip-based colorimetric device was developed for the determination of Cr³⁺ and Al³⁺ ions. The silver nanoparticles were modified with pyrrolidine-1-dithiocarboxylic acid ammonium salt as a novel ligand for the first time. The color of modified AgNPs in the test zone immediately changes after the addition of Cr³⁺ and Al³⁺ ions. The resulting color changes were detected by the naked eye or were taken by a smartphone camera. The obtained images were analyzed by RGB software to assay the Cr³⁺ and Al³⁺ ions concentration. Under optimized experimental conditions, the linear ranges are 0.1-220 and 0.01-250 µM for Cr³⁺ and Al³⁺ ions, respectively. The probe has a limit of detections of 10.66 and 3.55 nM for Cr³⁺ and Al³⁺ in an aqueous solution. In the case of µ-chip, the concentration ranges are 0.1-200 μM and 0.01-220 μM for Cr³⁺ and Al³⁺ ions, with detection limits of 9.18 and 2.30 nM, respectively. The µ-chip showed great potential as a fast detection tool for the monitoring of Cr³⁺ and Al³⁺ ions in real samples such as river water samples.

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.

Lab-on-Paper Devices for Diagnosis of Human Diseases Using Urine Samples-A Review

In recent years, microfluidic lab-on-paper devices have emerged as a rapid and low-cost alternative to traditional laboratory tests. Additionally, they were widely considered as a promising solution for point-of-care testing (POCT) at home or regions that lack medical infrastructure and resources. This review describes important advances in microfluidic lab-on-paper diagnostics for human health monitoring and disease diagnosis over the past five years. The review commenced by explaining the choice of paper, fabrication methods, and detection techniques to realize microfluidic lab-on-paper devices. Then, the sample pretreatment procedure used to improve the detection performance of lab-on-paper devices was introduced. Furthermore, an in-depth review of lab-on-paper devices for disease measurement based on an analysis of urine samples was presented. The review concludes with the potential challenges that the future development of commercial microfluidic lab-on-paper platforms for human disease detection would face.

Smartphone-enabled field monitoring tool for rapid hexavalent chromium detection in water

Chromium contamination of soil and water is a serious environmental and public health concern as the hexavalent form of chromium [Cr(VI)] is readily soluble in water and is a confirmed carcinogen. There is an imminent need for a robust, low-cost, and simple analytical technique to facilitate in situ monitoring of Cr(VI) in water. Current quantitative methods of Cr(VI) detection are largely laboratory-based, time-consuming, expensive, and require training for implementation. In this contribution, a portable, easy-to-use, and compact measuring tool is presented that provides Cr(VI) concentration within 10 min of water sampling over a linear range of 0-3 mg L^-1. This tool utilizes a relatively inexpensive camera-enabled smartphone with a custom-made test chamber attachment to seamlessly perform Cr(VI) measurements on water samples in the field. For analysis, an android-based software application was developed that directs the user to perform a simple series of steps following the diphenylcarbazide-based colorimetric method prescribed by the American Public Health Association. The tool was validated against a standard UV-visible spectrophotometer for a variety of synthetic and naturally contaminated water samples, with correlation factors greater than 0.993 (p < .001). The colorimetric method was also validated against a non-colorimetric Cr(VI) detection technique-ion chromatography-inductively coupled plasma mass spectrometry. Furthermore, Cr(VI) detection limits for the smartphone-enabled colorimetric method were found to be within 1.3-11.6 μg L^-1, which were significantly better than reported for commercially available field test kits, and even surpassed the limits exhibited by a typical spectrophotometer (25-50 μg L^-1). Finally, real-time mapping of source waters at a contaminated site was demonstrated by remote logging of Cr(VI) water quality data and corresponding GPS coordinates into a cloud server. This study highlights the potential role of smartphone-based monitoring tool in providing information to the affected community and enabling safe access to drinking water. An accurate, robust, simple-to-use, and economic method to measure hexavalent chromium in water within 10 min of sampling at site.

The colorimetric and microfluidic paper-based detection of cysteine and homocysteine using 1,5-diphenylcarbazide-capped silver nanoparticles

We have prepared a microfluidic paper-based analytical device (μPAD) for the determination of cysteine and homocysteine based on 1,5-diphenylcarbazide-capped silver nanoparticles. The μPAD was developed to identify and quantify the levels of cysteine and homocysteine. The proposed μPAD enabled the detection of cysteine and homocysteine using a colorimetric reaction based on modified silver nanoparticles. The color of the modified AgNPs in the test zone immediately changed after the addition of cysteine and homocysteine. Based on this change, the quantification of these two amino acids was achieved using an RGB color model and ImageJ software. Under optimized conditions, the proposed device enabled the determination of cysteine in the 0.20–20.0 μM concentration range with a limit of detection (LOD) of 0.16 μM. In addition, the LOD of homocysteine was calculated to be 0.25 μM with a linear range of 0.50–20.0 μM. In this work, we focused on the use of the μPAD for the analysis of a series of human urine samples.

A simple and novel portable method for the quantitative measurement of cysteine and homocysteine in human urine samples is presented.

Recent Advances in Microfluidic Paper-Based Analytical Devices toward High-Throughput Screening

Microfluidic paper-based analytical devices (µPADs) have become promising tools offering various analytical applications for chemical and biological assays at the point-of-care (POC). Compared to traditional microfluidic devices, µPADs offer notable advantages; they are cost-effective, easily fabricated, disposable, and portable. Because of our better understanding and advanced engineering of µPADs, multistep assays, high detection sensitivity, and rapid result readout have become possible, and recently developed µPADs have gained extensive interest in parallel analyses to detect biomarkers of interest. In this review, we focus on recent developments in order to achieve µPADs with high-throughput capability. We discuss existing fabrication techniques and designs, and we introduce and discuss current detection methods and their applications to multiplexed detection assays in relation to clinical diagnosis, drug analysis and screening, environmental monitoring, and food and beverage quality control. A summary with future perspectives for µPADs is also presented.

Microfluidic paper-based analytical device using gold nanoparticles modified with N,N′-bis(2-hydroxyethyl)dithiooxamide for detection of Hg(ii) in air, fish and water samples

A new method for visual detection of mercury by color change is developed that can detect Hg²⁺ by the naked eye or a digital camera.

A review on advances in methods for modification of paper supports for use in point-of-care testing

Paper is a widely used support for use in devices for point-of-care testing (POCT) in clinical diagnosis, food safety monitoring and environmental pollution. Paper is inexpensive, biocompatible, biodegradable and allows a sample fluid to flow by capillary force. Numerous method have been developed recently for chemical modification of papers in order to introduce different functionalities. This review (with 148 refs.) summarizes the recent progress in paper-based POCT devices. The introduction summarizes the state of the art of paper-based POCT devices and the physicochemical properties of existing unmodified materials (including cellulose, cellulose-based composites, cotton fibers, glass fibers, nitrocellulose, thread). Methods for paper modification for sample pretreatment are summarized next, with subsections on sample storage and collection, sample separation, nucleic acid extraction and sample preconcentration. Another main section covers approaches for paper modification for improving POCTs, with subsections on assays for proteins, nucleic acids, drugs, ion and organic molecules. The advantages and disadvantages of these approaches are compared. Several tables are presented that summarize the various modification techniques. A concluding section summarizes the current status, addresses challenges and gives an outlook on future perspectives of POCTs. Graphical abstract This review summarizes the progress that has been made in paper based point-of-care testing (POCT) and lateral flow assays (LFAs), quite often by using advanced nanomaterials for paper modification.

A label-free nano-probe for sequential and quantitative determination of Cr(VI) and ascorbic acid in real samples based on S and N dual-doped carbon dots

A fluorescent sulfur and nitrogen dual-doped carbon dots (S,N-CDs) was prepared by a simple and one-step acid-base neutralization and exothermic carbonization method. Hexavalent chromium (Cr(VI)) could effectively quench the fluorescence of S,N-CDs based on inner filter effect (IFE) and dynamic quenching, whereas ascorbic acid (AA) could recover the fluorescence of S,N-CDs/Cr(VI) because of IFE weakening. So an "on-off-on" and label-free nano-probe consecutive determination of Cr(VI) and AA was constructed. This nano-probe system demonstrated excellent selectivity and sensitivity to Cr(VI) and AA with linear range of 0.065-198 μmol/L (3.38-10,296 μg/L) and 6.6-892 μmol/L (1.16-157 mg/L), respectively. Meanwhile, the as-prepared S,N-CDs possess low toxicity and could be used for multi-color cell imaging in SMMC 7721 cells. More importantly, this nano-probe was successfully employed for detection of Cr(VI) in tap water and AA in food samples. In view of its simple detection condition, rapid response, wide linear range, low detection limit and inexpensive instrument, the as-constructed nano-probe system could have a wide range of potential application, including water quality monitoring and evaluation, food inspection and testing and biomedical analysis.

A fluorometric paper test for chromium(VI) based on the use of N-doped carbon dots

Water-soluble nitrogen-doped carbon quantum dots (C-dots) were fabricated by microwave-induced decomposition of the precursor materials citric acid and N,N'-bis(2-aminoethyl)-1,2-ethanediamine. The C-dots were placed on portable paper strips with novel origami designs to simplified user operations. The intensity of the blue fluorescence, best measured at excitation/emission wavelengths of 330/420 nm, depends on the pH value in the range from pH 2 to 12. The C-dots on the paper stripe are shown to be a sensitive fluorescent probe for chromium(VI) via an inner filter effect. Response is linear in the 0.08 to 1 mM concentration range, and the detection limit (at S/N = 3) is 0.14 mM. The test was applied to the determination of chromium(VI) in (spiked) environmental water samples. Graphical abstract Schematic presentation of the water-soluble nitrogen-doped carbon dots (C-dots) as a fluorescent probe for Cr⁶⁺ based on an inner filter effect. The three-dimensional paper analytical device integrating C-dots was applied to the determination of Cr⁶⁺ in (spiked) environmental water samples.