A portable and quantitative biosensor for cadmium detection using glucometer as the point-of-use device

New sensing methods using commercially available products: Based on PGM and PTS

In recent years, detection technology has made remarkable progress in the field of food safety, in vitro diagnosis, and environment monitoring under the impetus of trace substances detection requirements. However, in sharp contrast to the rapid development of detection technology, its marketization process is relatively lagging behind. One possible approach is to integrate novel sensing strategies with mature commercial devices, such as personal glucose meters (PGMs) and pregnancy test strips (PTS) to speed up their marketization process. In this review, we systematically summarized design principle, evolution, and application progress for the integration of novel sensing strategies with commercial devices PGMs and PTS. Meanwhile, key factors and difficulties for the integration novel sensing strategies with commercial devices were emphasized. More importantly, the future of prospects and remaining challenges were discussed.

Adsorptive Removal of Cd(II) Ions from Water by a Cheap Lignocellulosic Adsorbent and Its Reuse as a Catalyst for the Decontamination of Sulfamethoxazole

The work reports the removal of cadmium from water by applying an efficient low-cost lignocellulosic adsorbent, rooibos tea waste. The cadmium-loaded rooibos tea waste was used for the photocatalytic abatement of sulfamethoxazole to cater to the setback of secondary pollution mostly associated with the adsorption technique. The rooibos tea waste adsorbent displayed a high removal efficiency of about 90.63% for 10 mg/L Cd(II) ions at 45 °C, 180 min agitation time, pH 7, and a dosage of 500 mg. The process of Cd(II) adsorption was endothermic and spontaneous. Also, the spent adsorbent was found to be efficient toward the photocatalytic breakdown of 10 mg/L sulfamethoxazole with a degradation efficiency of 69% after 150 min. In addition, the extent of mineralization of the sulfamethoxazole by the spent adsorbent as obtained from the total organic carbon data was found to be 53%. Therefore, based on the results obtained from this work, rooibos tea waste lends itself as a cheap, eco-friendly, easily sourced, and viable adsorbent for the removal of toxic ions like Cd(II). Also, the successful reuse of the spent adsorbent is a promising approach to cater to the major setback of secondary pollution associated with adsorption technology.

Microvolumetric determination of thrombomodulin based on competitive immunoreaction using a portable glucometer

A new method of reducing the amount of reagent and sample for determination of thrombomodulin (TM) was developed based on competitive immunoreaction using a portable glucometer (PGM). Two types of nanocomposites, TM protein-modified magnetic nanoparticles (MNPs-TM) and TM antibody-/glucose oxidase-modified gold nanoparticles (Ab-GNPs-GOx), were prepared. Their binding product, MNPs-TM-Ab-GNPs-GOx, in the microvolumetric solution was used to catalyze the oxidation of glucose, leading to a decline of the glucose content. The TM-involved competitive immunoreaction had a negative effect on the generation of MNPs-/GNPs-based nanocomposites and inhibited the catalytic oxidation of glucose. The glucose content difference in the microvolumetric solution, which was revealed by a PGM, was in proportion to the logarithm of the TM concentration from 25 ng mL-1 to 2.5 μg mL-1. The limit of detection was 5.7 ng mL-1. Microvolumetric solution and a PGM were used in the measurement, which overcame some deficiencies of classical methods in chemo/biosensing, for example, special instrument, complicated measurement procedure, and high cost.

A novel β-cyclodextrin-assisted enhancement strategy for portable and sensitive detection of miR-21 in human serum

Benefiting from our discovery that β-cyclodextrin (β-CD) could enhance the catalytic activity of invertase through hydrogen bonding to improve detection sensitivity, a highly sensitive and convenient biosensor for the detection of miR-21 was proposed, which is based on the simplicity of reading signals from a personal glucose meter (PGM), combined with self-assembled signal amplification probes and the performance of β-CD as an enhancer. In the presence of miR-21, magnetic nanoparticle coupled capture DNA (MNPs-cDNA) could capture it and then connect assist DNA/H1-invertase (aDNA/H1) and self-assembled signal amplification probes (H1/H2) in turn. As a result, a "super sandwich" structure was formed. The invertase on MNPs-cDNA could catalyze the hydrolysis of sucrose to glucose and this catalytic process could be enhanced by β-CD. The PGM signal exhibited a linear correlation with miR-21 concentration within the range of 25 pmol L-1 to 3 nmol L-1, and the detection limit was as low as 5 pmol L-1 with high specificity. Moreover, the recoveries were 103.82-124.65% and RSD was 2.59-6.43%. Furthermore, the biosensor was validated for the detection of miR-21 in serum, and the results showed that miR-21 levels in serum samples from patients with Diffuse Large B-Cell Lymphoma (DLBCL) (n = 12) were significantly higher than those from healthy controls (n = 12) (P < 0.001). Therefore, the ingenious combination of PGM-based signal reading, self-assembled signal amplification probes and β-CD as an enhancer successfully constructed a convenient, sensitive and specific biosensing method, which is expected to be applied to clinical diagnosis.

Recent Advances in Personal Glucose Meter-Based Biosensors for Food Safety Hazard Detection

Food safety has emerged as a significant concern for global public health and sustainable development. The development of analytical tools capable of rapidly, conveniently, and sensitively detecting food safety hazards is imperative. Over the past few decades, personal glucose meters (PGMs), characterized by their rapid response, low cost, and high degree of commercialization, have served as portable signal output devices extensively utilized in the construction of biosensors. This paper provides a comprehensive overview of the mechanism underlying the construction of PGM-based biosensors, which consists of three fundamental components: recognition, signal transduction, and signal output. It also detailedly enumerates available recognition and signal transduction elements, and their modes of integration. Then, a multitude of instances is examined to present the latest advancements in the application of PGMs in food safety detection, including targets such as pathogenic bacteria, mycotoxins, agricultural and veterinary drug residues, heavy metal ions, and illegal additives. Finally, the challenges and prospects of PGM-based biosensors are highlighted, aiming to offer valuable references for the iterative refinement of detection techniques and provide a comprehensive framework and inspiration for further investigations.

A turn-on signal biosensor for cadmium(II) based on DNAzyme and stem-loop qPCR

Cadmium is a heavy metal that is exceedingly hazardous to humans and can enter the body through tainted food or drink, causing severe harm. It is critical to develop a technology for detecting cadmium in food and water that is sensitive and accurate. One such approach, which employs nucleases, is uncommon. A cadmium(II) turn-on biosensor was successfully created in this work using repetitive cleavage of certain specific nucleases for signal conversion and sophisticated stem-loop qPCR (quantitative polymerase chain reaction) for quick signal amplification and output. The method has strong selectivity and sensitivity for precise quantification, with a detection limit of 6 nmol L-1, i.e. 0.948 g L-1, which is far lower than the 5.0 g L-1 set by the United States Environmental Protection Agency, and it also operates well in retail rice samples.

A novel DNA-quantum dot nanostructure electrochemiluminescence aptamer sensor by chain reaction amplification for rapid detection of trace Cd2

This work proposes a new enzyme-free electrochemiluminescence (ECL) sensing platform based on a novel DNA-quantum dot (QD) nanostructure and hybridization chain reaction (HCR) amplification for the trace detection of Cd2+. First, the Cd2+ aptamer triggers the HCR amplification circuit, so abundant biotin-labeled DNAs are introduced to the electrode, and then biotin as a linker specifically captures a large number of streptavidin (SA)-CdS QD complexes, showing very high ECL signals. After the present Cd2+ binds to its aptamer on the electrode, it causes the linear DNA structure loaded with a large number of QDs to break away from the electrode, resulting in a significantly decreased ECL response. This method combines the HCR-amplified DNA structure-QD signal with the specificity of the biotin-avidin reaction, enabling the rapid detection of Cd2+ in complex water. Therefore, this sensor provides a novel and competitive strategy for detecting heavy metal ions in actual samples, which extends its application to practical settings, such as environmental monitoring and biomedical diagnostics.

Fluorescent aptasensor for the ultrasensitive detection of antibiotic residue in food samples based on dumbbell DNA-mediated signal amplification

Sensitive and reliable detection of antibiotics is of great significance for environmental and food safety due to its high risk in trace concentrations. Herein, we developed a fluorescence sensing system for chloramphenicol (CAP) detection based on dumbbell DNA-mediated signal amplification. Two hairpin dimers (2H1 and 2H2) were employed as the building blocks to construct the sensing scaffolds. The CAP-aptamer binding in another hairpin H0 can liberate the trigger DNA, which then activates the cyclic assembly reaction between 2H1 and 2H2. The separation of FAM and BHQ in the formed product of cascaded DNA ladder yields a high fluorescence signal for CAP monitoring. Compared with the monomer hairpin assembly between H1 and H2, the dimer hairpin assembly between 2H1 and 2H2 exhibits enhanced signal amplification efficiency and reduced reaction time. The developed CAP sensor showed a wide linear range from 10 fM to 10 nM with a detection limit of 2 fM. Importantly, this sensing platform has been successfully applied to the determination of CAP in fish, milk, and water samples with satisfactory recovery and accuracy. With the advantages of high sensitivity, mix-and-read pattern, and robustness, our proposed CAP sensor can be used as a simple and routine tool for the detection of trace amounts of antibiotic residues.

Portable biosensor for on-site detection of kanamycin in water samples based on CRISPR-Cas12a and an off-the-shelf glucometer

On-site and cost-effective monitoring of antibiotic residue in water samples using a ubiquitous device that is readily available to the general public is a big challenge. Herein, we developed a portable biosensor for kanamycin (KAN) detection based on a glucometer and CRISPR-Cas12a. The aptamer-KAN interactions liberate the trigger C strand, which can initiate the hairpin assembly to produce numerous double-stranded DNA. After recognition by CRISPR-Cas12a, Cas12a can cleave the magnetic bead and invertase-modified single-stranded DNA. After magnetic separation, the invertase can convert sucrose into glucose, which can be quantified by a glucometer. The linear range of the glucometer biosensor is from 1 pM to 100 nM and the detection limit is 1 pM. The biosensor also exhibited high selectivity and the nontarget antibiotics had no significant interference with KAN detection. The sensing system is robust and can work in complex samples with excellent accuracy and reliability. The recovery values were in the range of 89-107.2 % for water samples and 86-106.5 % for milk samples. The relative standard deviation (RSD) was below 5 %. With the advantages of simple operation, low cost, and easy accessibility to the public, this portable pocket-sized sensor can realize the on-site detection of antibiotic residue in resource-limited settings.

Personal Glucose Meters Coupled with Signal Amplification Technologies for Quantitative Detection of Non-Glucose Targets: Recent Progress and Challenges in Food Safety Hazards Analysis

Ensuring food safety is paramount worldwide. Developing effective detection methods to ensure food safety can be challenging owing to trace hazards, long detection time, and resource-poor sites, in addition to the matrix effects of food. Personal glucose meter (PGM), a classic point-of-care testing device, possesses unique application advantages, demonstrating promise in food safety. Currently, many studies have used PGM-based biosensors and signal amplification technologies to achieve sensitive and specific detection of food hazards. Signal amplification technologies have the potential to greatly improve the analytical performance and integration of PGMs with biosensors, which is crucial for solving the challenges associated with the use of PGMs for food safety analysis. This review introduces the basic detection principle of a PGM-based sensing strategy, which consists of three key factors: target recognition, signal transduction, and signal output. Representative studies of existing PGM-based sensing strategies combined with various signal amplification technologies (nanomaterial-loaded multienzyme labeling, nucleic acid reaction, DNAzyme catalysis, responsive nanomaterial encapsulation, and others) in the field of food safety detection are reviewed. Future perspectives and potential opportunities and challenges associated with PGMs in the field of food safety are discussed. Despite the need for complex sample preparation and the lack of standardization in the field, using PGMs in combination with signal amplification technology shows promise as a rapid and cost-effective method for food safety hazard analysis.

A Catechol-Meter Based on Conventional Personal Glucose Meter for Portable Detection of Tyrosinase and Sodium Benzoate

In this study, the personal glucose meter (PGM) was first used as a fast and user-friendly meter for analyzing catechol (CA) based on the reduction of the mediator K₃[Fe(CN)₆] to K₄[Fe(CN)₆] in the glucose test strip. Then, an easy, low-cost, and convenient PGM-based method for detecting tyrosinase (TYR) activity and sodium benzoate (SBA) was developed on the basis of the TYR-catalyzed reaction. In this method, CA is oxidized to form o-benzoquinone by TYR, thereby reducing the residual amount of CA and the PGM readout. On the other hand, SBA can inhibit the oxidation of CA catalyzed by TYR and increase the residual amount of CA after the enzymatic reaction. Therefore, the activity of TYR is proportional to the difference in the PGM readout of CA, and the concentration of SBA is positively correlated with the residual amount of CA. After the relevant experimental conditions were systematically optimized, the proposed PGM-based method for the detection of TYR and SBA was successfully validated. The liner ranges are 1.0-103.3 U/mL and 6.25-1000 ppm, and the quantification limits are 1.0 U/mL and 6.25 ppm for TYR and SBA, respectively. Moreover, the spiked recovery tests in normal human serum and carbonate beverages (i.e., Cola, Sprite, and Fanta) were performed, and the recoveries (91.6-106.8%) further confirm the applicability of the PGM-based method in real sample analysis.

A Simple and Universal Nucleic Acid Assay Platform Based on Personal Glucose Meter Using SARS-CoV-2 N Gene as the Model

A simple, selective, and quantitative platform for point-of-care diagnostic of COVID-19 is urgently needed as a complement in areas where resources are currently relatively scarce. To meet the needs of early diagnosis and intervention, a proof-of-concept demonstration of a universal personal glucose meter-based nucleic acid assay platform (PGM-NAAP) is presented, which converts to SARS-CoV-2 detection from glucose detection. By using magnetic bead separation together with the hand-held PGM for quantitative readout, PGM-NAAP achieves the 98 pM limit of detection for a sequence related to SARS-CoV-2. The ability to discriminate target nucleic acid from genomic DNA, the satisfactory spike recoveries of saliva and serum samples, as well as the good stability all together suggest the potential of the PGM-NAAP for the screening and diagnosis of suspected patients during the outbreaks of COVID-19 in resource-limited settings without sophisticated instruments. On the basis of these findings, PGM-NAAP can be expected to provide an accurate and convenient path for diagnosis of disease-associated nucleic acid.

Highly sensitive biosensor based on aptamer and hybridization chain reaction for detection of cadmium ions

In this work, a highly sensitive biosensor for detecting cadmium ions (Cd²⁺ ) was developed based on Cd²⁺ -specific DNA aptamer and hybridization chain reaction (HCR). The Cd²⁺ -aptamer (named S0) was used to recognize Cd²⁺ and trigger HCR reaction. Without Cd²⁺ , S0 initiated the HCR to form long nicked dsDNA structures to quench the fluorescence. Then, Cd²⁺ can bind with S0 to block HCR to recover fluorescence. This biosensor had high sensitivity with the detection limit of 0.36 nM and a linear range from 0 to 10 nM. Moreover, it showed a satisfactory selectivity and recovery rates.

A portable and quantitative detection of microRNA-21 based on cascade enzymatic reactions with dual signal outputs

MicroRNAs (miRNAs) are physiological status-related molecules which can be used as biomarkers for diseases, such as cancers. The point-of-care testing (POCT) of miRNAs has great application potential in early diagnosis and process monitoring of diseases. In this paper, a fast and dual signal outputs detection for microRNA-21 (miRNA-21) was established by using both personal glucose meter (PGM) and fluorescence spectrometer. In such an assay protocol, a dual-functional hairpin structure was rationally designed to recognize miRNA-21 and serve as the carrier of the reporter adenosine monophosphate (AMP). The hairpin structure can be specifically degraded by exonuclease T (Exo T) after hybridization with the target miRNA-21, releasing a large amount of AMP as the reporter. Then a smart signal conversion machinery composed of four enzymes and the corresponding substrates was employed to produce dual output signals through enzymatic cascade reactions. The machinery includes two parts: an adenosine triphosphate (ATP) generation system and a glucose consumption/NADPH production system. The produced AMP in the former step triggers the production of ATP, and subsequently the consumption of glucose and the production of NADPH. The changes of both glucose and NADPH are proportional to the concentration of miRNA-21, and can be determined by PGM and fluorescence spectrometer, respectively. Besides, the build-in substrate-recycling mechanism achieves signal amplification of the cascade enzymatic reactions. Under the optimal experimental conditions, the PGM signal is linearly correlated with the concentration of miRNA-21 in the range from 5 to 150 nM, with the limit of detection (LOD) of 3.65 nM. The LOD of fluorescence detection mode is even lowered to 0.03 nM. The miRNA-21-spiked serum samples, as well as the actual serum samples from cancer patients, have been successfully detected by this detection strategy. Thus the established assay provides a POCT solution for cancer diagnosis and prognosis.

Advances and Future Perspective on Detection Technology of Human Norovirus

Human norovirus (HuNoV) is a food-borne pathogen that causes acute gastroenteritis in people of all ages worldwide. However, no approved vaccines and antiviral drugs are available at present. Therefore, the development of accurate and rapid detection technologies is important in controlling the outbreak of HuNoVs. This paper reviewed the research progress on HuNoV detection, including immunological methods, molecular detection and biosensor technology. Immunological methods and molecular detection technologies are still widely used for HuNoV detection. Furthermore, biosensors will become an emerging developmental direction for the rapid detection of HuNoVs because of their high sensitivity, low cost, easy operation and suitability for onsite detection.

Portable, quantitative, and sequential monitoring of copper ions and pyrophosphate based on a DNAzyme-Fe3O4 nanosystem and glucometer readout

In this report, portable, quantitative, and sequential monitoring of copper ions and pyrophosphate (PPi) with a single sensor based on a DNAzyme-Fe₃O₄ system and glucometer readout was performed. Initially, streptavidin was functionalized on the surface of magnetic Fe₃O₄ spheres through glutaraldehyde. Then, an invertase-modified DNA Cu substrate was connected to the magnetic Fe₃O₄ spheres by a specific reaction between streptavidin and biotin. The sensing system was formed by a hybridization reaction between the Cu substrate and Cu enzyme. In the presence of Cu²⁺, Cu²⁺ will recognize the Cu DNA substrate and form an “off-on” signal switch, thereby resulting in the separation of invertase from the Fe₃O₄ nanospheres. PPi recognizes Cu²⁺ to form a Cu²⁺-PPi complex, resulting in an “on-off” signal switch. Under optimized conditions, linear detection ranges for Cu²⁺ and PPi of 0.01–5 and 0.5–10 μM, and detection limits for Cu²⁺ and PPi of 10 nM and 500 nM, respectively, were obtained. Good selectivity was achieved for the analysis of Cu²⁺ and PPi. Satisfactory results were achieved for this biosensor during the determination of Cu²⁺ in real tap samples and PPi in human urine samples. This verified that the sensor is portable and low cost, and can be applied to the sequential monitoring of multiple analytes with a single point-of-care biosensor.

Biosensing with DNAzymes

This article provides a comprehensive review of biosensing with DNAzymes, providing an overview of different sensing applications while highlighting major progress and seminal contributions to the field of portable biosensor devices and point-of-care diagnostics. Specifically, the field of functional nucleic acids is introduced, with a specific focus on DNAzymes. The incorporation of DNAzymes into bioassays is then described, followed by a detailed overview of recent advances in the development of in vivo sensing platforms and portable sensors incorporating DNAzymes for molecular recognition. Finally, a critical perspective on the field, and a summary of where DNAzyme-based devices may make the biggest impact are provided.

Nucleic Acid Tests for Clinical Translation

Nucleic acids, including deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are natural biopolymers composed of nucleotides that store, transmit, and express genetic information. Overexpressed or underexpressed as well as mutated nucleic acids have been implicated in many diseases. Therefore, nucleic acid tests (NATs) are extremely important. Inspired by intracellular DNA replication and RNA transcription, in vitro NATs have been extensively developed to improve the detection specificity, sensitivity, and simplicity. The principles of NATs can be in general classified into three categories: nucleic acid hybridization, thermal-cycle or isothermal amplification, and signal amplification. Driven by pressing needs in clinical diagnosis and prevention of infectious diseases, NATs have evolved to be a rapidly advancing field. During the past ten years, an explosive increase of research interest in both basic research and clinical translation has been witnessed. In this review, we aim to provide comprehensive coverage of the progress to analyze nucleic acids, use nucleic acids as recognition probes, construct detection devices based on nucleic acids, and utilize nucleic acids in clinical diagnosis and other important fields. We also discuss the new frontiers in the field and the challenges to be addressed.

Advances in aptamer screening and aptasensors' detection of heavy metal ions

Heavy metal pollution has become more and more serious with industrial development and resource exploitation. Because heavy metal ions are difficult to be biodegraded, they accumulate in the human body and cause serious threat to human health. However, the conventional methods to detect heavy metal ions are more strictly to the requirements by detection equipment, sample pretreatment, experimental environment, etc. Aptasensor has the advantages of strong specificity, high sensitivity and simple preparation to detect small molecules, which provides a new direction platform in the detection of heavy metal ions. This paper reviews the selection of aptamers as target for heavy metal ions since the 21th century and aptasensors application for detection of heavy metal ions that were reported in the past five years. Firstly, the selection methods for aptamers with high specificity and high affinity are introduced. Construction methods and research progress on sensor based aptamers as recognition element are also introduced systematically. Finally, the challenges and future opportunities of aptasensors in detecting heavy metal ions are discussed.

A Facile Aptasensor for Instantaneous Determination of Cadmium Ions Based on Fluorescence Amplification Effect of MOPS on FAM-Labeled Aptamer

Analytical performance and efficiency are two pivotal issues for developing an on-site and real-time aptasensor for cadmium (Cd²⁺) determination. However, suffering from redundant preparations, fabrications, and incubation, most of them fail to well satisfy the requirements. In this work, we found that fluorescence intensity of 6-carboxyfluorescein(FAM)-labeled aptamer (FAM-aptamer) could be remarkably amplified by 3-(N-morpholino)propane sulfonic acid (MOPS), then fell proportionally as Cd²⁺ concentration introduced. Importantly, the fluorescence variation occurred immediately after addition of Cd²⁺, and would keep stable for at least 60 min. Based on the discovery, a facile and ultra-efficient aptasensor for Cd²⁺ determination was successfully developed. The sensing mechanism was confirmed by fluorescence pattern, circular dichroism (CD) and intermolecular interaction related to pK_a. Under the optimal conditions, Cd²⁺ could be determined rapidly from 5 to 4000 ng mL^-1. The detection limit (1.92 ng mL^-1) was also lower than the concentration limit for drinking water set by WHO and EPA (3 and 5 ng mL^-1, respectively). More than a widely used buffer, MOPS was firstly revealed to have fluorescence amplification effect on FAM-aptamer upon a given context. Despite being sensitive to pH, this simple, high-performance and ultra-efficient aptasensor would be practical for on-site and real-time monitoring of Cd²⁺.

Sensitive and Specific Cadmium Biosensor Developed by Reconfiguring Metal Transport and Leveraging Natural Gene Repositories

Whole-cell biosensors are useful for monitoring heavy metal toxicity in public health and ecosystems, but their development has been hindered by intrinsic trade-offs between sensitivity and specificity. Here, we demonstrated an effective engineering solution by building a sensitive, specific, and high-response biosensor for carcinogenic cadmium ions. We genetically programmed the metal transport system of Escherichia coli to enrich intracellular cadmium ions and deprive interfering metal species. We then selected 16 cadmium-sensing transcription factors from the GenBank database and tested their reactivity to 14 metal ions in the engineered E. coli using the expression of the green fluorescent protein as the readout. The resulting cadmium biosensor was highly specific and showed a detection limit of 3 nM, a linear increase in fluorescent intensities from 0 to 200 nM, and a maximal 777-fold signal change. Using this whole-cell biosensor, a smartphone, and low-tech equipment, we developed a simple assay capable of measuring cadmium ions at the same concentration range in irrigation water and human urine. This method is user-friendly and cost-effective, making it affordable to screen large amounts of samples for cadmium toxicity in agriculture and medicine. Moreover, our work highlights natural gene repositories as a treasure chest for bioengineering.

DNAzyme Amplified Aptasensing Platform for Ochratoxin A Detection Using a Personal Glucose Meter

Aptamer-based sensors have emerged as a major platform for detecting small-molecular targets, because aptamers can be selected to bind these small molecules with higher affinity and selectivity than other receptors such as antibodies. However, portable, accurate, sensitive, and affordable detection of these targets remains a challenge. In this work, we developed an aptasensing platform incorporating magnetic beads and a DNAzyme for signal amplification, resulting in high sensitivity. The biosensing platform was constructed by conjugating a biotin-labeled aptamer probe of small-molecular targets such as toxins and a biotin-labeled substrate strand on magnetic beads, and the DNAzyme strand hybridized with the aptamer probe to block the substrate cleavage activity. The specific binding of the small-molecular target by the aptamer probe can replace the DNAzyme strand and then induce the hybridization between the DNAzyme strand and substrate strand, and the iterative signal amplification reaction of hydrolysis and cleavage of the substrate chain occurs in the presence of a metal ion cofactor. Using invertase to label the substrate strand, the detection of small molecules of the toxin is successfully transformed into the measurement of glucose, and the sensitive analysis of small molecules such as toxins can be realized by using the household portable glucose meter as a readout. This platform is shown to detect ochratoxin, a common toxin in food, with a linear detection range of 5 orders of magnitude, a low detection limit of 0.88 pg/mL, and good selectivity. The platform is easy to operate and can be used as a potential choice for quantitative analysis of small molecules, at home or under point-of-care settings. Moreover, by changing and designing the aptamer probe and the arm of DNAzyme strand, it can be used for the analysis of other analytes.

Recent Advances in Nanotechnology-Based Biosensors Development for Detection of Arsenic, Lead, Mercury, and Cadmium

Heavy metals cause considerable environmental pollution due to their extent and non-degradability in the environment. Analysis and trace levels of arsenic, lead, mercury, and cadmium as the most toxic heavy metals show that they can cause various hazards in humans' health. To achieve rapid, high-sensitivity methods for analyzing ultra-trace amounts of heavy metals in different environmental and biological samples, novel biosensors have been designed with the participation of strategies applied in nanotechnology. This review attempted to investigate the novel, sensitive, efficient, cost-benefit, point of care, and user-friendly biosensors designed to detect these heavy metals based on functional mechanisms. The study's search strategies included examining the primary databases from 2015 onwards and various keywords focusing on heavy metal biosensors' performance and toxicity mechanisms. The use of aptamers and whole cells as two important bio-functional nanomaterials is remarkable in heavy metal diagnostic biosensors' bioreceptor design. The application of hybridized nanomaterials containing a specific physicochemical function in the presence of a suitable transducer can improve the sensing performance to achieve an integrated detection system. Our study showed that in addition to both labeled and label-free detection strategies, a wide range of nanoparticles and nanocomposites were used to modify the biosensor surface platform in the detection of heavy metals. The detection limit and linear dynamic range as an essential characteristic of superior biosensors for the primary toxic metals are studied. Furthermore, the perspectives and challenges facing the design of heavy metal biosensors are outlined. The development of novel biosensors and the application of nanotechnology, especially in real samples, face challenges such as the capability to simultaneously detect multiple heavy metals, the interference process in complex matrices, the efficiency and stability of nanomaterials implemented in various laboratory conditions.

ZnO-reduced graphene oxide composite based photoelectrochemical aptasensor for sensitive Cd(II) detection with methylene blue as sensitizer

In this paper a photoelectrochemical (PEC) aptasensor based on specific recognition with conformational changed after the target Cd(II) identification was fabricated. A ZnO and reduced graphene oxide (ZnO-rGO) nanocomposite with enhanced PEC activity was designed as photoactive material. After the further incorporation of gold nanoparticles (AuNPs) with ZnO-rGO nanocomposite, the enhanced photocurrent signal could be detected owing to the localized surface plasmon resonance and good conductivity of AuNPs. In addition, AuNPs were used as anchors for immobilization of -SH modified aptamer S1. After that aptamer S2 was paired with S1 sequence to form complementary double stranded DNA (dsDNA) on the electrode surface. Methylene blue (MB) was acted as sensitizer and assembled in dsDNA structure to amplify photocurrent response. When Cd(II) was bound to the aptamer presented on the sensing interface, S2 specifically recognized and captured Cd(II), which resulted in the unwinding of dsDNA structure and the separation of MB molecules from the electrode surface with photocurrent response decreased. The photocurrent was detected by a double-working-electrode system, which used the modified electrode as the first working electrode and glassy carbon electrode (GCE) as the second working electrode. Dopamine (DA) was added to the electrolyte and acted as the electron donor, which could be oxidized on the modified electrode and reduced on the GCE to form a cyclic reaction, leading to the enhanced photocurrent response with improved photocurrent stability. This MB sensitized PEC aptasensor exhibited a high sensitivity with a detection limit of 1.8 × 10^-12 mol/L (3σ). Thus, a highly sensitive aptasensor with double-working-electrode detection method for Cd(II) determination were established and further applied to the water samples analysis.

Versatile Sensing Platform for Cd2+ Detection in Rice Samples and Its Applications in Logic Gate Computation

A versatile sensing platform was designed for Cd²⁺ detection utilizing Mg²⁺-dependent DNAzyme as the biocatalyst and toehold-mediated strand replacement as the reaction mechanism. The Cd²⁺-aptamer interaction brings the split subunits of the Mg²⁺-dependent DNAzyme into close-enough proximity, which generates an active DNAzyme that can catalyze the cleavage reaction toward the hairpin substrate strand (H1). The trigger DNA fragment in H1 can open another hairpin probe (H2) to activate the cyclic signal amplification process. The generated numerous G-quadruplex DNAzyme structures will produce a high fluorescence response after incubation with the fluorescence dye N-methyl mesoporphyrin IX (NMM). This detection platform is ultrasensitive and the detection limit (LOD) is 2.5 pM (S/N = 3). The sensing system is robust and can work effectively even in a complex sample matrix, enabling the quantitative analysis of Cd²⁺ content in rice samples with good reliability. Showing the unique features of simple operation, label-free and enzyme-free format, high sensitivity and selectivity, and universal signal amplification mode, our proposed sensing protocol holds great promise for becoming a competitive alternative for the routine monitoring of Cd²⁺ pollution. Importantly, this flexible and versatile sensing platform was used to construct some exquisite logic gates, including AND, OR, INHIBIT, IMPLICATION, NOR, and NAND.

Translating in vitro diagnostics from centralized laboratories to point-of-care locations using commercially-available handheld meters

There is a growing demand for high-performance point-of-care (POC) diagnostic technologies where in vitro diagnostics (IVD) is fundamental for prevention, identification, and treatment of many diseases. Over the past decade, a shift of IVDs from the centralized laboratories to POC settings is emerging. In this review, we summarize recent progress in translating IVDs from centralized labs to POC settings using commercially available handheld meters. After introducing typical workflows for IVDs and highlight innovative technologies in this area, we discuss advantages of using commercially available handheld meters for translating IVDs from centralized labs to POC settings. We then provide comprehensive coverage of different signal transduction strategies to repurpose the commercially-available handheld meters, including personal glucose meter, pH meter, thermometer and pressure meter, for detecting a wide range of targets by integrating biochemical assays with the meters for POC testing. Finally, we identify remaining challenges and offer future outlook in this area.

Ammonium Pyrrolidine Dithiocarbamate-Modified CdTe/CdS Quantum Dots as a Turn-on Fluorescent Sensor for Detection of Trace Cadmium Ions

In this work, ammonium pyrrolidine dithiocarbamate (APDC) was used as a surface etchant to modify CdTe/CdS core-shell quantum dots (QDs). The APDC etchant combines with the cadmium ions (Cd²⁺) on the surface of the QDs, resulting in the formation of surface holes. The formation of these holes changes the QD surface structure, which leads to fluorescence quenching of the QDs. Newly added Cd²⁺ can selectively recognize and combine with these holes; thus, the fluorescence intensity of the QDs can be restored. The linear response of this turn-on fluorescent sensor was found to be 0-100 μg/L and 100-600 μg/L under the determined optimal conditions, and its limit of detection (LOD) for Cd²⁺ was 2.642 μg/L (23.5 nmol/L).

An enzyme-free DNA circuit for the amplified detection of Cd2+ based on hairpin probe-mediated toehold binding and branch migration

An enzyme-free DNA circuit was designed for the amplified detection of Cd2+ based on hairpin probe-mediated toehold binding and branch migration. A Cd2+-specific aptamer was used to recognize Cd2+ and a G-quadruplex was used to report the detection signal. The assay is sensitive, with a detection limit of 5 pM.