Electrochemical Aptamer-Based Biosensors for Measurements in Undiluted Human Saliva

Y-Shaped Deoxyribonucleic Acid Scaffold Pendulums: A One-Step Electrochemical Sensor

The challenge of developing sensing platforms for the direct monitoring of targets within complex samples is well-recognized. To address this, a one-step electrochemical sensing detection platform was introduced, featuring an innovative Y-shaped DNA molecular pendulum design. The approach deviated from the conventional molecular pendulum mode by employing a split aptamer instead of a full one, thereby enabling the detection of small molecules and low-molecular-weight proteins. Three Y-shaped DNA molecular pendulum configurations were designed: the single-arm, the flexible double-arm, and the stable double-arm Y-shaped DNA molecular pendulum. The results revealed that the Y-shaped scaffold pendulum with a stable two-armed structure not only offered a broader detection range for target concentrations but also produced a more substantial electrical signal enhancement compared to other modes. This enhanced performance is attributed to the stable conformation of this design, which prolongs the time the probe takes to overcome fluid resistance and reach the electrode surface, leading to a more significant alteration in the electrical signal. The sensor can be utilized for one-step detection of enrofloxacin (ENR) in diluted samples (milk, artificial urine, and cosmetics), and its detection range (0.001-100 ng/mL) is fully compliant with the EU maximum residue levels (100 ng/mL) for ENR in milk. Additionally, the sensor can detect myoglobin (Myo) in artificial urine and serum by simply changing the recognized DNA strand. This work provided a simple, expandable idea for the detection of small molecules and low-molecular-weight proteins.

Continuous High-Throughput Plasma Separation for Blood Biomarker Sensing Using a Hydrodynamic Microfluidic Device

Continuous, cost-effective, high-throughput with admissible yield and purity of blood plasma separation is widely needed for biomarker detection in the clinic. The existing gold standard technique (centrifugation) and microfluidic technologies fall short of meeting these criteria. In this study, a microfluidic device design is demonstrated based on passive hydrodynamic principles to achieve admissible yield and purity plasma samples. Through computational and experimental assessments, it is shown that side channels with varying lengths are required to improve the plasma extraction rate. The optimized side channels in this device design use the formed cell-free layer regions in the expanded areas to extract plasma consistently and efficiently. These Hydrodynamic Continuous, High-Throughput Plasma Separator (HCHPS) microfluidic devices achieve a purity in the range of 47% to 64% with whole blood and maintaining a yield of 10% to 18%, with half hemolysis compared to gold standard centrifugation. These devices also separate the plasma from diluted blood with a purity in the range of 62% to 97% with a similar yield range. Additionally, whole human blood spiked with lactate was processed through the HCHPS device, and the separated plasma is collected and analyzed using two biosensing approaches, a bead-based fluorescence, and an electrochemical aptamer biosensing, confirming the quality of plasma for downstream biomarker detection.

Single-Use Electrochemical Aptamer-Based Sensors for Calibration-Free Measurements in Human Saliva via Dual-Frequency Approaches: Prospects and Challenges

Despite the rapid growth in aptamer-based biosensor research, there remains a significant demand for aptasensors that operate without the need for sample preparation and calibration, to better facilitate real-world applications. Electrochemical aptamer-based (EAB) sensors, particularly those utilizing a dual-frequency, calibration-free approach, have shown promising advances toward commercialization. Single-use, disposable sensors represent a cost-effective solution for at-home and on-site point-of-care (POC) diagnostics. However, the development of these sensors presents unique challenges compared to in vivo monitoring and reusable platforms, with pronounced variations across sensors and batches. Motivated by these challenges, we have comprehensively investigated the dual-frequency, calibration-free approach, focusing on sensor-to-sensor and batch-to-batch variations. Our research explored the use of a nonresponsive frequency-based ratiometric method for detecting cocaine with laser-ablated, disposable EAB sensors. Additionally, to overcome the absence of nonresponsive frequencies in some aptasensors, we developed strategies to modify the aptamer structure and optimize operational conditions, effectively tailoring nonresponsive frequencies to allow for rapid result turnover. Moreover, we assessed the effects of various filter types on saliva pretreatment using liquid chromatography with tandem mass spectrometry (LCMS/MS) and developed a saliva collection workflow using an oral swab. This workflow and the disposable aptasensors developed herein achieved low μM sensitivity in saliva, with results obtainable in under 5 min, including saliva collection and processing. Furthermore, our findings indicate that certain food and drink residues in saliva can compromise sensor accuracy, highlighting an area for future refinement.

Design of Label-Free DNA Light-Up Aptaswitches for Multiplexed Biosensing

We present a straightforward design approach to develop DNA-based light-up aptasensors. We performed the first systematic comparison of DNA fluorescent light-up aptamers (FLAPs), revealing key differences in affinity and specificity for their target dyes. Based on our analysis, two light-up aptamers emerged with remarkable specificity, fluorescence enhancement, and functionality in diverse environments. We then established generalizable design rules to couple the DNA FLAPs to small molecule-binding aptamers, creating 13 novel aptaswitches with reliable turn-on or turn-off aptaswitching in a dose-response manner. We developed new aptaswitches for ochratoxin A and ATP biosensing with up to a seven-fold response and low background. Finally, we demonstrated the orthogonal activity of our aptaswitch platforms. As a result, we introduce fluorescent light-up aptaswitches for one-pot detection of different targets in diverse sample matrices.

Electrochemical Aptamer-Based Biosensors for Cocaine Detection in Human Saliva: Exploring Matrix Interference

Electrochemical aptamer-based biosensors (E-aptasensors) are emerging platforms for point-of-care (POC) detection of complex biofluids. Human saliva particularly offers a noninvasive matrix and unprecedented convenience for detecting illicit drugs, such as cocaine. However, the sensitivity of cocaine E-aptasensors is significantly compromised in saliva. Herein, we investigated the influence of salivary components on the sensing performance of a methylene blue (MB)-labeled classic cocaine aptamer by square-wave voltammetry (SWV), and in parallel, we report the development and optimization of a disposable E-aptasensor for cocaine detection fabricated by laser ablation. Cyclic voltammetry (CV), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to study the cleanliness and surface topography of the disposable electrode surface. To enhance the sensing performance of the disposable platform, we developed a co-immobilization strategy by introducing both the target and 6-mercapto-1-hexanol (MCH) into the aptamer immobilization solution, achieving optimal sensing performance at the aptamer-to-MCH ratio of 1:100. In a buffer solution, we revealed that the aptasensor performs best at low ionic strength, the absence of multivalent ions, and neutral pH conditions, while salivary components such as viscosity and mucin have minimal impact. However, upon transition to human saliva, the presence of salivary proteins exerted a profound effect on the sensing performance. To reduce this impact, we discovered that a high NaCl concentration could significantly enhance the sensing response in saliva. This approach circumvents centrifugation and extensive dilution and facilitates cocaine detection in human saliva through a straightforward "mix-and-detect" method. This disposable aptasensor achieved a limit of detection (LOD) of 3.7 μM in 90% saliva, demonstrating immense promise for the application of electrochemical aptasensors in detecting cocaine, especially when administered via smoking.

Wearable Electrochemical Biosensors for Advanced Healthcare Monitoring

Recent advancements in wearable electrochemical biosensors have opened new avenues for on-body and continuous detection of biomarkers, enabling personalized, real-time, and preventive healthcare. While glucose monitoring has set a precedent for wearable biosensors, the field is rapidly expanding to include a wider range of analytes crucial for disease diagnosis, treatment, and management. In this review, recent key innovations are examined in the design and manufacturing underpinning these biosensing platforms including biorecognition elements, signal transduction methods, electrode and substrate materials, and fabrication techniques. The applications of these biosensors are then highlighted in detecting a variety of biochemical markers, such as small molecules, hormones, drugs, and macromolecules, in biofluids including interstitial fluid, sweat, wound exudate, saliva, and tears. Additionally, the review also covers recent advances in wearable electrochemical biosensing platforms, such as multi-sensory integration, closed-loop control, and power supply. Furthermore, the challenges associated with critical issues are discussed, such as biocompatibility, biofouling, and sensor degradation, and the opportunities in materials science, nanotechnology, and artificial intelligence to overcome these limitations.

Truncations and in silico docking to enhance the analytical response of aptamer-based biosensors

Aptamers are short oligonucleotides capable of binding specifically to various targets (i.e., small molecules, proteins, and whole cells) which have been introduced in biosensors such as in the electrochemical aptamer-based (E-AB) sensing platform. E-AB sensors are comprised of a redox-reporter-modified aptamer attached to an electrode that undergoes, upon target addition, a binding-induced change in electron transfer rates. To date, E-AB sensors have faced a limitation in the translatability of aptamers into the sensing platform presumably because sequences obtained from Systematic Evolution of Ligands by Exponential Enrichment (SELEX) are typically long (>80 nucleotides) and that obtaining structural information remains time and resource consuming. In response, we explore the utility of aptamer base truncations and in silico docking to improve their translatability into E-AB sensors. Here, we first apply this to the glucose aptamer, which we characterize in solution using NMR methods to guide design and translate truncated variants in E-AB biosensors. We further investigated the applicability of the truncation and computational approaches to four other aptamer systems (vancomycin, cocaine, methotrexate and theophylline) from which we derived functional E-AB sensors. We foresee that our strategy will increase the success rate of translating aptamers into sensing platforms to afford low-cost measurements of molecules directly in undiluted complex matrices.

Ligand-Induced Folding in a Dopamine-Binding DNA Aptamer

Aptamers are often employed as molecular recognition elements in the development of different types of biosensors. Many of these biosensors take advantage of the aptamer having a ligand-induced structure-formation binding mechanism. However, this binding mechanism is poorly understood. Here we use isothermal titration calorimetry, circular dichroism spectroscopy and NMR spectroscopy to study the binding and ligand-induced structural change exhibited by a dopamine-binding DNA aptamer. We analysed a series of aptamers where we shorten the terminal stem that contains the 5' and 3' termini of the aptamer sequence. All aptamers bind dopamine in an enthalpically driven process coupled with an unfavorable entropy. A general trend of the aptamer having a weaker binding affinity is observed as the terminal stem is shortened. For all aptamers studied, numerous signals appear in the imino region of the 1H NMR spectrum indicating that new structure forms with ligand binding. However, it is only when this region of structure formation in the aptamer is brought close to the sensor surface that we obtain a functional electrochemical aptamer-based biosensor.

Aptamer-Mediated Electrochemical Detection of SARS-CoV-2 Nucleocapsid Protein in Saliva

Gold standard detection of SARS-CoV-2 by reverse transcription quantitative PCR (RT-qPCR) can achieve ultrasensitive viral detection down to a few RNA copies per sample. Yet, the lengthy detection and labor-intensive protocol limit its effectiveness in community screening. In view of this, a structural switching electrochemical aptamer-based biosensor (E-AB) targeting the SARS-CoV-2 nucleocapsid (N) protein was developed. Four N protein-targeting aptamers were characterized on an electrochemical cell configuration using square wave voltammetry (SWV). The sensor was investigated in an artificial saliva matrix optimizing the aptamer anchoring orientation, SWV interrogation frequency, and target incubation time. Rapid detection of the N protein was achieved within 5 min at a low nanomolar limit of detection (LOD) with high specificity. Specific N protein detection was also achieved in simulated positive saliva samples, demonstrating its feasibility for saliva-based rapid diagnosis. Further research will incorporate novel signal amplification strategies to improve sensitivity for early diagnosis.

A Wearable Electrochemical Sensor Based on a Molecularly Imprinted Polymer Integrated with a Copper Benzene-1,3,5-Tricarboxylate Metal-Organic Framework for the On-Body Monitoring of Cortisol in Sweat

Owing to their potential to transform traditional medical diagnostics and health monitoring, wearable biosensors have become an alternative evolutionary technology in the field of medical care. However, it is still necessary to overcome some key technique challenges, such as the selectivity, sensitivity, and stability of biometric identification. Herein, a novel, wearable electrochemical sensor based on a molecularly imprinted polymer (MIP) integrated with a copper benzene-1,3,5-tricarboxylate metal-organic framework (MOF) was designed for the detection of stress through the on-body monitoring of cortisol in sweat. The MOF was used as the substrate for MIP deposition to enhance the stability and sensitivity of the sensor. The sensor consisted of two layers, with a microfluidic layer as the top layer for spontaneous sweating and a modified electrode as the bottom layer for sensing. The sensor measured cortisol levels by detecting the current change that occurred when the target molecules bound to the imprinted cavities, using Prussian blue nanoparticles embedded in the MIP framework as the REDOX probe. The proposed sensor exhibited a linear detection range of 0.01-1000 nM with a detection limit of 0.0027 nM, and favorable specificity over other analogies. This facile anti-body free sensor showed excellent stability, and can be successfully applied for in situ cortisol monitoring.

Electrochemical sensing mechanisms of neonicotinoid pesticides and recent progress in utilizing functional materials for electrochemical detection platforms

The excessive residue of neonicotinoid pesticides in the environment and food poses a severe threat to human health, necessitating the urgent development of a sensitive and efficient method for detecting trace amounts of these pesticides. Electrochemical sensors, characterized by their simplicity of operation, rapid response, low cost, strong selectivity, and high feasibility, have garnered significant attention for their immense potential in swiftly detecting trace target molecules. The detection capability of electrochemical sensors primarily relies on the catalytic activity of electrode materials towards the target analyte, efficient loading of biomolecular functionalities, and the effective conversion of interactions between the target analyte and its receptor into electrical signals. Electrode materials with superior performance play a crucial role in enhancing the detection capability of electrochemical sensors. With the continuous advancement of nanotechnology, particularly the widespread application of novel functional materials, there is paramount significance in broadening the applicability and expanding the detection range of pesticide sensors. This comprehensive review encapsulates the electrochemical detection mechanisms of neonicotinoid pesticides, providing detailed insights into the outstanding roles, advantages, and limitations of functional materials such as carbon-based materials, metal-organic framework materials, supramolecular materials, metal-based nanomaterials, as well as molecular imprinted materials, antibodies/antigens, and aptamers as molecular recognition elements in the construction of electrochemical sensors for neonicotinoid pesticides. Furthermore, prospects and challenges facing various electrochemical sensors based on functional materials for neonicotinoid pesticides are discussed, providing valuable insights for the future development and application of biosensors for simplified on-site detection of agricultural residues.