Conformable surface acoustic wave biosensor for E-coli fabricated on PEN plastic film

A functionalized microwave biosensor for rapid, reagent-free detection of E. coli in water samples

Escherichia coli (E. coli) O157:H7 (O157), one of the most common Shiga toxin-producing E. coli, can contaminate water systems causing severe illnesses often accompanied with diarrhea and sometimes life threatening. Frequent monitoring of E. coli in water systems is critical to protect public health. Most traditional methods for E. coli detection are slow in responding to E. coli outbreaks due to the need for sample transportation from the site to the lab, expensive equipment, and highly trained personnel for the detection. This work presents a novel reagent-free detection method that employs a microwave biosensor functionalized with an antibody specific to E. coli to offer rapid and sensitive E. coli detection. By monitoring the resonance frequency shift caused by the binding between the E. coli in the water sample and the antibody coated on the sensor using a vector network analyzer (VNA), this microwave-based biosensor achieved a limit of detection (LOD) of 647 CFU/ml. This LOD can be further reduced to 6.47 CFU/ml with a simple preconcentration step prior to the sensing procedure. The sensor has also been tested to detect E. coli in natural water systems with a low-cost, palm-sized portable VNA, suggesting its excellent feasibility for real-time on-site E.coli detection.

A multimetallic nanozyme enhanced colorimetric biosensor for Salmonella detection on finger-actuated microfluidic chip

Salmonella screening is essential to avoid food poisoning. A simple, fast and sensitive colorimetric biosensor was elaborately developed for Salmonella detection on a microfluidic chip through limiting air chambers for precise air control, switching rotary valves for accurate fluid selection, a convergence-and-divergence passive micromixer and an extrusion-and-suction active micromixer for efficient fluid mixing, and immune gold@platinum palladium nanocatalysts for effective signal amplification. The mixture of bacteria, immune magnetic nanobeads and nanocatalysts was first rapidly mixed to form nanobead-bacteria-nanocatalyst conjugates and magnetically separated for enrichment. After washing with water, the conjugates were used to catalyze colorless substrate and blue product was finally analyzed using ImageJ for quantifying bacterial concentration. The finger-actuated microfluidic chip enabled designated control of designated fluids in designated places towards designated directions by simple press-release operations on designated air chambers without any external power. Under optimal conditions, this sensor could detect Salmonella at 45 CFU/mL in 25 min.

Frequency-locked Wireless Multifunctional Surface Acoustic Wave Sensors

Surface acoustic waves (SAWs) have shown great potential for developing sensors for structural health monitoring (SHM) and lab-on-a-chip (LOC) applications. Existing SAW sensors mainly rely on measuring the frequency shifts of high-frequency (e.g., >0.1 GHz) resonance peaks. This study presents frequency-locked wireless multifunctional SAW sensors that enable multiple wireless sensing functions, including strain sensing, temperature measurement, water presence detection, and vibration sensing. Our sensors leverage SAW resonators on piezoelectric chips, inductive coupling-based wireless power transmission, and, particularly, a frequency-locked wireless sensing mechanism that works at low frequencies (e.g., <0.1 GHz). This mechanism locks the input frequency on the slope of a sensor's reflection spectrum and monitors the reflection signal's amplitude change induced by the changes of sensing parameters. The proof-of-concept experiments show that our wireless sensors can operate in a low-power active mode for on-demand wireless strain measurement, temperature sensing, and water presence detection. Moreover, our sensors can operate in a power-free passive mode for vibration sensing, with results that agree well with laser vibrometer measurements. We anticipate that the designs and mechanisms of our frequency-locked wireless SAW sensors will inspire researchers to develop future wireless multifunctional sensors for SHM and LOC applications.

Foodborne pathogen detection using surface acoustic wave biosensors: a review

This paper summarizes several attractive surface acoustic wave (SAW) biosensors, including Love-wave sensors, dual-channel SAW sensors, langasite SAW sensors, and SAW syringe filters. SAW sensors with different piezoelectric materials and high-frequency SAW sensors used for identifying the food pathogenic bacteria Escherichia coli (E. coli) are discussed together with the examples of methods based on such sensing technology that have been effectively utilized in diagnostics and epidemiological research. This review also emphasizes some of the limitations of using these biosensors, which have prompted the increased need for more rapid, sensitive, selective, portable, power-efficient, and low-cost methods for detecting these pathogens. It is envisioned that SAW devices will have remarkable significance in the future.

A passive wireless surface acoustic wave (SAW) sensor system for detecting warfare agents based on fluoroalcohol polysiloxane film

Long-term monitoring of environmental warfare agengts is a challenge for chemical gas sensors. To address this issue, we developed a 433 MHz passive wireless surface acoustic wave (WSAW) gas sensor for dimethyl methylphosphonate (DMMP) detection. This WSAW gas sensor includes a YZ lithium niobate (LiNbO3) substrate with metallic interdigital transducers (IDTs) etched on it, and an antenna was placed near the IDT. A DMMP-sensitive viscoelastic polymer fluoroalcoholpolysiloxane (SXFA) film was prepared on a LiNbO3 substrate, and mode modeling coupling was used to optimize the design parameters. The sensor can function properly in an environments between -30 °C and 100 °C with humidity less than 60% RH. When the wireless transmission distance was within the range of 0-90 cm, the sensor noise increased with distance, and the stability was less than 32°/h. While optimizing the film thickness of SXFA, a relationship was observed between sensor sensitivity and film thickness. When the film thickness of SXFA reached 450 nm, the optimal value was reached. At a distance of 20 cm between the transmitting and receiving antennas, DMMP was detected at different concentrations with the developed WSAW gas sensor. The lower detection limit of DMMP was 0.48 mg/m3, the sensitivity of the sensor was 4.63°/(mg/m3), and repeatable performance of the sensor was confirmed.

Low-noise fluorescent detection of cardiac troponin I in human serum based on surface acoustic wave separation

Acute myocardial infarction (AMI) is a life-threatening disease when sudden blockage of coronary artery occurs. As the most specific biomarker, cardiac troponin I (cTnI) is usually checked separately to diagnose or eliminate AMI, and achieving the accurate detection of cTnI is of great significance to patients' life and health. Compared with other methods, fluorescent detection has the advantages of simple operation, high sensitivity and wide applicability. However, due to the strong fluorescence interference of biological molecules in body fluids, it is often difficult to obtain high sensitivity. In order to solve this problem, in this study, surface acoustic wave separation is designed to purify the target to achieve more sensitive detection performance of fluorescent detection. Specifically, the interference of background noise is almost completely removed on a microfluidic chip by isolating microbeads through acoustic radiation force, on which the biomarkers are captured by the immobilized detection probe. And then, the concentration of cTnI in human serum is detected by the fluorescence intensity change of the isolated functionalized beads. By this way, the detection limit of our biosensor calculated by 3σ/K method is 44 pg/mL and 0.34 ng/mL in PBS buffer and human serum respectively. Finally, the reliability of this method has been validated by comparison with clinical tests from the nephelometric analyzer in hospital.

Progress in wearable acoustical sensors for diagnostic applications

With extensive and widespread uses of miniaturized and intelligent wearable devices, continuously monitoring subtle spatial and temporal changes in human physiological states becomes crucial for daily healthcare and professional medical diagnosis. Wearable acoustical sensors and related monitoring systems can be comfortably applied onto human body with a distinctive function of non-invasive detection. This paper reviews recent advances in wearable acoustical sensors for medical applications. Structural designs and characteristics of the structural components of wearable electronics, including piezoelectric and capacitive micromachined ultrasonic transducer (i.e., pMUT and cMUT), surface acoustic wave sensors (SAW) and triboelectric nanogenerators (TENGs) are discussed, along with their fabrication techniques and manufacturing processes. Diagnostic applications of these wearable sensors for detection of biomarkers or bioreceptors and diagnostic imaging have further been discussed. Finally, main challenges and future research directions in these fields are highlighted.

Fully Microfabricated Surface Acoustic Wave Tweezer for Collection of Submicron Particles and Human Blood Cells

Precise manipulation of (sub)micron particles is key for the preparation, enrichment, and quality control in many biomedical applications. Surface acoustic waves (SAW) hold tremendous promise for manipulation of (bio)particles at the micron to nanoscale ranges. In commonly used SAW tweezers, particle manipulation relies on the direct acoustic radiation effect whose superior performance fades rapidly when progressing from micron to nanoscale particles due to the increasing dominance of a second order mechanism, termed acoustic streaming. Through reproducible and high-precision realization of stiff microchannels to reliably actuate the microchannel cross-section, here we introduce an approach that allows the otherwise competing acoustic streaming to complement the acoustic radiation effect. The synergetic effect of both mechanisms markedly enhances the manipulation of nanoparticles, down to 200 nm particles, even at relatively large wavelength (300 μm). Besides spherical particles ranging from 0.1 to 3 μm, we show collections of cells mixed with different sizes and shapes inherently existing in blood including erythrocytes, leukocytes, and thrombocytes.

Conventional and advanced detection techniques of foodborne pathogens: A comprehensive review

Foodborne pathogens are a major public health concern and have a significant economic impact globally. From harvesting to consumption stages, food is generally contaminated by viruses, parasites, and bacteria, which causes foodborne diseases such as hemorrhagic colitis, hemolytic uremic syndrome (HUS), typhoid, acute, gastroenteritis, diarrhea, and thrombotic thrombocytopenic purpura (TTP). Hence, early detection of foodborne pathogenic microbes is essential to ensure a safe food supply and to prevent foodborne diseases. The identification of foodborne pathogens is associated with conventional (e.g., culture-based, biochemical test-based, immunological-based, and nucleic acid-based methods) and advances (e.g., hybridization-based, array-based, spectroscopy-based, and biosensor-based process) techniques. For industrial food applications, detection methods could meet parameters such as accuracy level, efficiency, quickness, specificity, sensitivity, and non-labor intensive. This review provides an overview of conventional and advanced techniques used to detect foodborne pathogens over the years. Therefore, the scientific community, policymakers, and food and agriculture industries can choose an appropriate method for better results.

A precise and rapid early pregnancy test: Development of a novel and fully automated electrochemical point-of-care biosensor for human urine samples

Human chorionic gonadotropin (hCG), a glycoprotein hormone secreted from the placenta, is an important biomarker for pregnancy. In this study, we designed a precise, rapid and fully automatic device with an electrochemical point-of-care biosensor capable of quantitative hCG detection from human urine samples for early pregnancy detection. Gold and Ag/AgCl electrodes, whose structure with optimum isopotential region and current density, were simulated using COMSOL Multiphysics® software and custom-made from Flex Medical. The sensing surface was fabricated with DSP self-assembled monolayers (SAMs) and covalently immobilized anti-hCG-beta antibody. The detection method involved a sandwich assay using anti-hCG alpha-HRP. Based on an automated agitation design implemented in our device, the surface reaction rate is significantly improved comparing to routinely performed sandwich assays, and therefore a rapid detection of very low concentration can be achieved. Electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) measurements were used to characterize the immobilization of the antibodies and to determine the sensor activities respectively. The sensors displayed a limit of detection (LOD) of 2.17 mIU/ml within established clinical hCG levels for early detection of pregnancy. They responded very well to hCG, but not to luteinizing hormone (LH), which has a high degree of cross-reactivity with hCG. The results showed that the immunosensor has high specificity, good reproducibility, and long-term stability for the detection of hCG in urine samples.

Acoustofluidics - changing paradigm in tissue engineering, therapeutics development, and biosensing

For more than 70 years, acoustic waves have been used to screen, diagnose, and treat patients in hundreds of medical devices. The biocompatible nature of acoustic waves, their non-invasive and contactless operation, and their compatibility with wide visualization techniques are just a few of the many features that lead to the clinical success of sound-powered devices. The development of microelectromechanical systems and fabrication technologies in the past two decades reignited the spark of acoustics in the discovery of unique microscale bio applications. Acoustofluidics, the combination of acoustic waves and fluid mechanics in the nano and micro-realm, allowed researchers to access high-resolution and controllable manipulation and sensing tools for particle separation, isolation and enrichment, patterning of cells and bioparticles, fluid handling, and point of care biosensing strategies. This versatility and attractiveness of acoustofluidics have led to the rapid expansion of platforms and methods, making it also challenging for users to select the best acoustic technology. Depending on the setup, acoustic devices can offer a diverse level of biocompatibility, throughput, versatility, and sensitivity, where each of these considerations can become the design priority based on the application. In this paper, we aim to overview the recent advancements of acoustofluidics in the multifaceted fields of regenerative medicine, therapeutic development, and diagnosis and provide researchers with the necessary information needed to choose the best-suited acoustic technology for their application. Moreover, the effect of acoustofluidic systems on phenotypic behavior of living organisms are investigated. The review starts with a brief explanation of acoustofluidic principles, the different working mechanisms, and the advantages or challenges of commonly used platforms based on the state-of-the-art design features of acoustofluidic technologies. Finally, we present an outlook of potential trends, the areas to be explored, and the challenges that need to be overcome in developing acoustofluidic platforms that can echo the clinical success of conventional ultrasound-based devices.

Recent advances of surface acoustic wave-based sensors for noninvasive cell analysis

In the past years, the application of surface acoustic waves (SAWs) as sensors for biological applications has reached high relevance in the field of biotechnology. From rapid advances in designs and materials, new opportunities have emerged, especially for sensing of living cells. Additionally, the combination of SAW sensors with microfluidics and optical microscopy has expanded the market of possible applications. Differentiation of infected and healthy red blood cells or aggressive and nonaggressive tumor cells, and monitoring of wound healing, bacteria, or viral antigen concentrations via SAW-based sensors are only a few examples of recent achievements in cell biology. The rapid growth of this field requires frequent reviewing of the recent progress to maintain high research standards and promote future developments.

Monitoring of Drug Release via Intra Body Communication with an Edible Pill

Oral drug administration provides a convenient and patient‐compliant way for drug delivery, especially for chronic diseases and prolonged pharmacological treatments. However, due to the repetitiveness of such therapeutic approach, the patients are led to neglect/forget the therapy affecting the healthcare delivery. Indeed, the non‐adherence to pharmacological prescriptions and the unknown amount of real‐time drug release result in a non‐compliant therapeutic drug level over the protracted therapies. The proposed technology will enable the monitoring of both pharmacological adherence and real‐time drug release. The approach exploits a passive intrabody communication (IBC) activation in order to enable an edible pill, realized starting from food additives and food‐grade materials, to monitor pharmacological adherence. Following activation, the signal is modulated by IBC coupling switching triggered by pill degradation in a gastrointestinal tract, resulting in a monitored drug release. The proof‐of‐concept is designed for a targeted release and monitoring of Metformin in the intestine. The system shows an in vitro limit of cumulative drug release detection of 18 µg mL−1 and a limit of real‐time drug release detection of 2 µg mL−1 min−1. This platform represents the first solution to monitor passive drug release in real‐time, from intake to complete absorption, enabling unique and long‐sought healthcare therapy and treatment opportunity.

Combining plasmon-enhanced fluorescence with Rayleigh surface acoustic waves to quantify Carcinoembryonic Antigen from human plasma

To improve the direct quantification of Carcinoembryonic Antigen (CEA) from body fluids by immunofluorescence, a surface acoustic wave (SAW) based biosensor was developed combined with an optimized silver nanostructure at the sensing region. Fluorescence signal amplification is achieved by patterning silver nanostructures using the rapid thermal annealing (RTA) method. In addition, the problem of background noise interference from nonspecific binding in human plasma is addressed by Rayleigh wave streaming at the immunoassay region, which shows a reduction in the limit of detection. The results show that the silver nanostructures significantly increase the sensor sensitivity by 49.99-fold and lower the limit of detection of CEA in phosphate buffered saline (PBS) solution to 101.94 pg/mL. The limit of detection of CEA biomarker in human plasma was successfully brought down to 11.81 ng/mL by reducing background noise using Rayleigh SAW streaming. This allows for a point-of-need sensor system to be realized in various clinical biosensing applications.

Advances, applications, and limitations of portable and rapid detection technologies for routinely encountered foodborne pathogens

Traditional foodborne pathogen detection methods are highly dependent on pre-treatment of samples and selective microbiological plating to reliably screen target microorganisms. Inherent limitations of conventional methods include longer turnaround time and high costs, use of bulky equipment, and the need for trained staff in centralized laboratory settings. Researchers have developed stable, reliable, sensitive, and selective, rapid foodborne pathogens detection assays to work around these limitations. Recent advances in rapid diagnostic technologies have shifted to on-site testing, which offers flexibility and ease-of-use, a significant improvement from traditional methods' rigid and cumbersome steps. This comprehensive review aims to thoroughly discuss the recent advances, applications, and limitations of portable and rapid biosensors for routinely encountered foodborne pathogens. It discusses the major differences between biosensing systems based on the molecular interactions of target analytes and biorecognition agents. Though detection limits and costs still need further improvement, reviewed technologies have high potential to assist the food industry in the on-site detection of biological hazards such as foodborne pathogens and toxins to maintain safe and healthy foods. Finally, this review offers targeted recommendations for future development and commercialization of diagnostic technologies specifically for emerging and re-emerging foodborne pathogens.

A Surface Acoustic Wave (SAW)-Based Lab-on-Chip for the Detection of Active α-Glycosidase

Enzyme detection in liquid samples is a complex laboratory procedure, based on assays that are generally time- and cost-consuming, and require specialized personnel. Surface acoustic wave sensors can be used for this application, overcoming the cited limitations. To give our contribution, in this work we present the bottom-up development of a surface acoustic wave biosensor to detect active α-glycosidase in aqueous solutions. Our device, optimized to work at an ultra-high frequency (around 740 MHz), is functionalized with a newly synthesized probe 7-mercapto-1-eptyl-D-maltoside, bringing one maltoside terminal moiety. The probe is designed ad hoc for this application and tested in-cuvette to analyze the enzymatic conversion kinetics at different times, temperatures and enzyme concentrations. Preliminary data are used to optimize the detection protocol with the SAW device. In around 60 min, the SAW device is able to detect the enzymatic conversion of the maltoside unit into glucose in the presence of the active enzyme. We obtained successful α-glycosidase detection in the concentration range 0.15-150 U/mL, with an increasing signal in the range up to 15 U/mL. We also checked the sensor performance in the presence of an enzyme inhibitor as a control test, with a signal decrease of 80% in the presence of the inhibitor. The results demonstrate the synergic effect of our SAW Lab-on-a-Chip and probe design as a valid alternative to conventional laboratory tests.

Flexible multifunctional platform based on piezoelectric acoustics for human-machine interaction and environmental perception

Flexible human-machine interfaces show broad prospects for next-generation flexible or wearable electronics compared with their currently available bulky and rigid counterparts. However, compared to their rigid counterparts, most reported flexible devices (e.g., flexible loudspeakers and microphones) show inferior performance, mainly due to the nature of their flexibility. Therefore, it is of great significance to improve their performance by developing and optimizing new materials, structures and design methodologies. In this paper, a flexible acoustic platform based on a zinc oxide (ZnO) thin film on an aluminum foil substrate is developed and optimized; this platform can be applied as a loudspeaker, a microphone, or an ambient sensor depending on the selection of its excitation frequencies. When used as a speaker, the proposed structure shows a high sound pressure level (SPL) of ~90 dB (with a standard deviation of ~3.6 dB), a low total harmonic distortion of ~1.41%, and a uniform directivity (with a standard deviation of ~4 dB). Its normalized SPL is higher than those of similar devices reported in the recent literature. When used as a microphone, the proposed device shows a precision of 98% for speech recognition, and the measured audio signals show a strong similarity to the original audio signals, demonstrating its equivalent performance compared to a rigid commercial microphone. As a flexible sensor, this device shows a high temperature coefficient of frequency of -289 ppm/K and good performance for respiratory monitoring.

A New Biorecognition-Element-Free IDμE Sensor for the Identification and Quantification of E. coli

The label-free biosensor has emerged as an effective tool for the purpose of early detection of causative pathogens such as Escherichia coli as a preventive measure. In this study, a biorecognition-element-free interdigitated microelectrode (IDμE) sensor is designed and developed with this in mind, with good reliability and affordability. Results show that the designed sensor can identify E. coli with good selectivity using an impedance and capacitance of 7.69 MHz. At its optimum impedance of 1.3 kHz, the IDμE sensor can reliably quantify E. coli in a range of measurement (10^3.2~10⁶ cfu/mL), linearity (R² = 0.97), sensitivity (18.15 kΩ/log (cfu/mL)), and limit of detection (10^3.2 cfu/mL). In summary, the IDμE sensor developed possesses high potential for industrial and clinical applications.

Wearable Sensors for Healthcare: Fabrication to Application

This paper presents a substantial review of the deployment of wearable sensors for healthcare applications. Wearable sensors hold a pivotal position in the microelectronics industry due to their role in monitoring physiological movements and signals. Sensors designed and developed using a wide range of fabrication techniques have been integrated with communication modules for transceiving signals. This paper highlights the entire chronology of wearable sensors in the biomedical sector, starting from their fabrication in a controlled environment to their integration with signal-conditioning circuits for application purposes. It also highlights sensing products that are currently available on the market for a comparative study of their performances. The conjugation of the sensing prototypes with the Internet of Things (IoT) for forming fully functioning sensorized systems is also shown here. Finally, some of the challenges existing within the current wearable systems are shown, along with possible remedies.

Recent Advances in Intelligent Wearable Medical Devices Integrating Biosensing and Drug Delivery

The primary roles of precision medicine are to perform real-time examination, administer on-demand medication, and apply instruments continuously. However, most current therapeutic systems implement these processes separately, leading to treatment interruption and limited recovery in patients. Personalized healthcare and smart medical treatment have greatly promoted research on and development of biosensing and drug-delivery integrated systems, with intelligent wearable medical devices (IWMDs) as typical systems, which have received increasing attention because of their non-invasive and customizable nature. Here, the latest progress in research on IWMDs is reviewed, including their mechanisms of integrating biosensing and on-demand drug delivery. The current challenges and future development directions of IWMDs are also discussed.

A Proof-of-Concept Electrochemical Cytosensor Based on Chlamydomonas reinhardtii Functionalized Carbon Black Screen-Printed Electrodes: Detection of Escherichia coli in Wastewater as a Case Study

Herein, we report a proof-of-concept algal cytosensor for the electrochemical quantification of bacteria in wastewater, exploiting the green photosynthetic alga Chlamydomonas reinhardtii immobilized on carbon black (CB) nanomodified screen-printed electrodes. The CB nanoparticles are used as nanomodifiers, as they are able to sense the oxygen produced by the algae and thus the current increases when algae are exposed to increasing concentrations of bacteria. The sensor was tested on both standard solutions and real wastewater samples for the detection Escherichia coli in a linear range of response from 100 to 2000 CFU/100 mL, showing a limit of detection of 92 CFU/100 mL, in agreement with the maximum E. coli concentration established by the Italian law for wastewater (less than 5000 CFU/100 mL). This bacterium was exploited as a case study target of the algal cytosensor to demonstrate its ability as an early warning analytical system to signal heavy loads of pathogens in waters leaving the wastewater treatment plants. Indeed, the cytosensor is not selective towards E. coli but it is capable of sensing all the bacteria that induce the algae oxygen evolution by exploiting the effect of their interaction. Other known toxicants, commonly present in wastewater, were also analyzed to test the cytosensor selectivity, with any significant effect, apart from atrazine, which is a specific target of the D1 protein of the Chlamydomonas photosystem II. However, the latter can also be detected by chlorophyll fluorescence simultaneously to the amperometric measurements. The matrix effect was evaluated, and the recovery values were calculated as 105 ± 8, 83 ± 7, and 88 ± 7% for 1000 CFU/100 mL of E. coli in Lignano, San Giorgio, and Pescara wastewater samples, respectively.

Microfluidic electrochemical magnetoimmunosensor for ultrasensitive detection of interleukin-6 based on hybrid of AuNPs and graphene

Cytokines are important factors in the early diagnosis of autoimmune diseases and require high sensitivity, high selectivity and quantitative detection. We proposed a miniaturized electrochemical magneto-immunosensor (EC-MIS) on portable interleukin-6 (IL-6) detection based on this requirement. Firstly, a micro-fabricated working electrode is electrochemically modified with a hybrid of reduced graphene oxide (rGO) and gold nanoparticles (AuNPs). Increased surface area and enhanced charge transfer rate improve the performance of this immunosensor on sensitivity. Secondly, magnetic beads attached with the capture antibody (cAb) are employed in sandwich immunoassay. This kind of immunoassay is immobilized on the working electrode surface by an external magnet to enrich the analyte IL-6. Thirdly, the last two features are combined and integrated on a microfluidic device in order to restrict the sample at certain areas and ease the operation of detection. With our prototypic EC-MIS operated in amperometric mode, we have achieved the detection of IL-6 with a linear range from 0.97 to 250 pg/mL and a limit of detection (LOD) of 0.42 pg/mL. Real serum samples were demonstrated and compared with benchtop equipment's results.

Acoustic Biosensors and Microfluidic Devices in the Decennium: Principles and Applications

Lab-on-a-chip (LOC) technology has gained primary attention in the past decade, where label-free biosensors and microfluidic actuation platforms are integrated to realize such LOC devices. Among the multitude of technologies that enables the successful integration of these two features, the piezoelectric acoustic wave method is best suited for handling biological samples due to biocompatibility, label-free and non-invasive properties. In this review paper, we present a study on the use of acoustic waves generated by piezoelectric materials in the area of label-free biosensors and microfluidic actuation towards the realization of LOC and POC devices. The categorization of acoustic wave technology into the bulk acoustic wave and surface acoustic wave has been considered with the inclusion of biological sample sensing and manipulation applications. This paper presents an approach with a comprehensive study on the fundamental operating principles of acoustic waves in biosensing and microfluidic actuation, acoustic wave modes suitable for sensing and actuation, piezoelectric materials used for acoustic wave generation, fabrication methods, and challenges in the use of acoustic wave modes in biosensing. Recent developments in the past decade, in various sensing potentialities of acoustic waves in a myriad of applications, including sensing of proteins, disease biomarkers, DNA, pathogenic microorganisms, acoustofluidic manipulation, and the sorting of biological samples such as cells, have been given primary focus. An insight into the future perspectives of real-time, label-free, and portable LOC devices utilizing acoustic waves is also presented. The developments in the field of thin-film piezoelectric materials, with the possibility of integrating sensing and actuation on a single platform utilizing the reversible property of smart piezoelectric materials, provide a step forward in the realization of monolithic integrated LOC and POC devices. Finally, the present paper highlights the key benefits and challenges in terms of commercialization, in the field of acoustic wave-based biosensors and actuation platforms.

Smart choices: Mechanisms of intelligent food packaging

Intelligent food packaging is usually designed to monitor the state of the food itself and/or the environment around it, as well as the interactions between them, providing customers with information on food quality and/or safety through a variety of signals. They involve indicators (which inform by direct visual changes about specific properties related to food quality) and sensors (which detect specific analytes by using receptors, transducers, and signal processing electronics). A third type of intelligent packaging is known as data carriers, which are not typically used for information on food quality, but rather to track the movement of food along the food supply chain. In this graphical review, the basic mechanisms of intelligent food packaging systems are presented, as well as their main applications, with particular emphasis on those focused on food quality monitoring.

Flexible thin-film acoustic wave devices with off-axis bending characteristics for multisensing applications

Flexible surface acoustic wave (SAW) devices have recently attracted tremendous attention for their widespread application in sensing and microfluidics. However, for these applications, SAW devices often need to be bent into off-axis deformations between the acoustic wave propagation direction and bending direction. Currently, there are few studies on this topic, and the bending mechanisms during off-axis bending deformations have remained unexplored for multisensing applications. Herein, we fabricated aluminum nitride (AlN) flexible SAW devices by using high-quality AlN films deposited on flexible glass substrates and systematically investigated their complex deformation behaviors. A theoretical model was first developed using coupling wave equations and the boundary condition method to analyze the characteristics of the device with bending and off-axis deformation under elastic strains. The relationships between the frequency shifts of the SAW device and the bending strain and off-axis angle were obtained, and the results were identical to those from the theoretical calculations. Finally, we performed proof-of-concept demonstrations of its multisensing potential by monitoring human wrist movements at various off-axis angles and detecting UV light intensities on a curved surface, thus paving the way for the application of versatile flexible electronics.

Sensitivity Enhancement in Plasma Polymer Films for Surface Acoustic Wave Based Sensor Applications

Plasma polymer films (PPF), widely used as sensing layers in surface acoustic wave (SAW) based gas and liquid phase sensors, have a major drawback: high concentrations of the sensed analytes easily drive these films into saturation, where accurate measurements are no longer possible. This work suggests a solution to this problem by modifying the PPF with the sensed chemical compound to improve the overall sorption properties and sensor dynamic range. Thin polymer films were synthesized from hexamethyldisiloxane (HMDSO) and triethylsilane (TES) monomers in a plasma-enhanced chemical vapor deposition (PECVD) process using a RF plasma reactor. We used these Si-containing compounds because they are known for their excellent sensing properties. In this work, the layers were deposited onto the active surface of high-Q 438 MHz Rayleigh SAW two-port resonators, used as mass sensitive sensor elements. We call these devices quartz surface microbalances (QSM). In a second step, ammonia plasma modification was applied to the HMDSO and TES films, in order to achieve a higher sensitivity to NH3. The sensors were probed at different NH3 gas concentrations in a computer controlled gas probing setup. A comparison with unmodified films revealed a 74% to 85% improvement in both the sensitivity and sorption ability of the HMDSO sensing layers, and of about 8% for the TES films.

Piezoelectric Smart Patch Operated with Machine-Learning Algorithms for Effective Detection and Elimination of Condensation

Timely detection and elimination of surface condensation is crucial for diverse applications in agriculture, automotive, oil and gas industries, and respiratory monitoring. In this paper, a smart patch based on a ZnO/aluminum (∼5 μm/50 μm thick) flexible Lamb wave device has been proposed to detect, prevent, and eliminate condensation, which can be realized using both of its surfaces. The patch is operated using a machine-learning algorithm which consists of data preprocessing (feature selection and optimization) and model training by a random forest algorithm. It has been tested in six cases, and the results show good detection performance with average precision = 94.40% and average F1 score = 93.23%. The principle of accelerating evaporation is investigated to understand the elimination and prevention functions for surface condensation. Results show that both dielectric heating and acoustothermal effect have their contributions, whereas the former is found more dominant. Furthermore, the functional relationship between the evaporation rate and the input power is calibrated, showing a high linearity (R² = 97.64%) with a slope of ∼3.6 × 10^-5 1/(s·mW). With an input power of ∼0.6 W, the flexible device has been proven effective in the prevention of condensation.

Flexible SAW Microfluidic Devices as Wearable pH Sensors Based on ZnO Nanoparticles

In this work, a new flexible and biocompatible microfluidic pH sensor based on surface acoustic waves (SAWs) is presented. The device consists of polyethylene naphthalate (PEN) as a flexible substrate on which aluminum nitride (AlN) has been deposited as a piezoelectric material. The fabrication of suitable interdigitated transducers (IDTs) generates Lamb waves (L-SAW) with a center frequency ≈500 MHz traveling in the active region. A SU-8 microfluidics employing ZnO nanoparticles (NPs) functionalization as a pH-sensitive layer is fabricated between the IDTs, causing a shift in the L-SAW resonance frequency as a function of the change in pH values. The obtained sensitivity of ≈30 kHz/pH from pH 7 to pH 2 demonstrates the high potential of flexible SAW devices to be used in the measurement of pH in fluids and biosensing.

Regenerable ZnO/GaAs Bulk Acoustic Wave Biosensor for Detection of Escherichia coli in "Complex" Biological Medium

A regenerable bulk acoustic wave (BAW) biosensor is developed for the rapid, label-free and selective detection of Escherichia coli in liquid media. The geometry of the biosensor consists of a GaAs membrane coated with a thin film of piezoelectric ZnO on its top surface. A pair of electrodes deposited on the ZnO film allows the generation of BAWs by lateral field excitation. The back surface of the membrane is functionalized with alkanethiol self-assembled monolayers and antibodies against E. coli. The antibody immobilization was investigated as a function of the concentration of antibody suspensions, their pH and incubation time, designed to optimize the immunocapture of bacteria. The performance of the biosensor was evaluated by detection tests in different environments for bacterial suspensions ranging between 10³ and 10⁸ CFU/mL. A linear dependence between the frequency response and the logarithm of E. coli concentration was observed for suspensions ranging between 10³ and 10⁷ CFU/mL, with the limit of detection of the biosensor estimated at 10³ CFU/mL. The 5-fold regeneration and excellent selectivity towards E. coli detected at 10⁴ CFU/mL in a suspension tinted with Bacillus subtilis at 10⁶ CFU/mL illustrate the biosensor potential for the attractive operation in complex biological media.

Ultra-sensitive photoelectrochemical aptamer biosensor for detecting E. coli O157:H7 based on nonmetallic plasmonic two-dimensional hydrated defective tungsten oxide nanosheets coupling with nitrogen-doped graphene quantum dots (dWO3•H2O@N-GQDs)

Light absorption and interfacial engineering of photoactive materials play vital roles in photoexcited electron generation and electron transport, and ultimately boost the performance of photoelectrochemical (PEC) biosensing. In this work, a novel high-performance photoelectrochemical (PEC) biosensing platform was fabricated based on nonmetallic plasmonic tungsten oxide hydrate nanosheets (WO₃•H₂O) coupling with nitrogen doped graphene quantum dots (N-GQDs) by a facile one-step hydrothermal approach. The localized surface plasmon resonance (LSPR) properties were achieved by oxygen vacancy engineered WO₃·H₂O (dWO₃•H₂O), which could greatly extend the light absorption from visible light to near-infrared light. Moreover, by coupling with N-GQDs, the as-fabricated heterojunction (dWO₃•H₂O@N-GQD) provided a much enhanced photoelectric response due to the efficient charge transfer. By conjugation with E.coli O157:H7 aptamer, a novel PEC aptasensor based on dWO₃•H₂O@N-GQD heterojunction was fabricated with a high sensitivity for detection of E.coli O157:H7. The limit of detection (LOD) of this PEC aptasensor is 0.05 CFU/mL with a linear detection range from 0.1 to 10⁴ CFU/mL. Moreover, high reproducibility and good accuracy could also be achieved for analysis in milk samples. This work could provide a promising platform for the development of PEC bioanalysis and offer an insight into the non-metallic plasmonic materials based heterojunctions for high-performances PEC biosensing.

Recent advances in acoustic wave biosensors for the detection of disease-related biomarkers: A review

In the past several decades, acoustic wave biosensors, as an emerging kind of biosensors, have been developed and widely used for the detection of mass, viscosity, conductivity and density. Varieties of applications have been explored such as medical diagnosis, drug screening, environmental monitoring, food analysis and biochemical assay. Among them, the detection of disease-related biomarkers based on acoustic sensors has aroused great research interest all over the world. In this review, the classification and characteristics of acoustic wave biosensors are briefly introduced. Then, some classical studies and recent advances in disease-related biomarker detection utilizing these biosensors are summarized and detailed, respectively. Here, the disease-related biomarkers mainly include antigens, small molecular proteins, cancer cells, viruses and VOCs. Finally, challenges and future trends of these typical acoustic wave biosensors are discussed. Compared with other reviews of acoustic wave sensors, this review highlights the great potential of typical acoustic wave biosensors for early disease screening and diagnosis compared with widely-used medical imaging. Moreover, they are integrated with other technologies for the design of multi-analyte, multi-parameter and intelligent devices, collecting more comprehensive information from biomarkers. This review provides a new perspective on the applications and optimization of acoustic wave biosensors to develop more reliable platforms for disease-related biomarker detection and disease diagnosis.

Use of Nanoparticle Enhanced Phase Change Material for Cooling of Surface Acoustic Wave Sensor

The use of resonators, filters, interdigital transducers (IDT) and stable sources in electronic industry is widespread today. One of the most used filters are the surface acoustic wave (SAW) type, which is mostly based on Rayleigh waves propagation on the surface. On the other hand, the use of Phase change materials (PCMs) is considered as a heat sink method in the field of thermal cooling of electronic circuits. Recent development in heat transfer is obtained by nanoparticle-enhanced PCM (NEPCM), which is a result of combining nanoparticles with PCMs. Increase of thermal conductivity of NEPCM in comparison with common PCM enhances the heat transfer rate. The aim of the current study is thermal management of SAW for the application of high frequency heating by phase change material. Melting of NEPCMs inside a rectangular cavity next to the SAW cell is used for the cooling purpose. Free convection heat transfer of a NEPCMs in an square cavity is modeled throughout the mass and momentum. Energy governing equations are solved by using the finite element method. Electrohydrodynamic (EHD) forces exist in natural convection heat transfer within the fluid part of the enclosure. The results also show that the NEPCM causes heat transfer improvement up to 10%.

Multifunctional nanoplatform for dual-mode sensitive detection of pathogenic bacteria and the real-time bacteria inactivation

Bacterial infection is a growing public health concern and causes a huge medical and financial burden. It is of significance to efficiently construct multifunctional platforms for bacterial point-of-care testing (POCT) and elimination. Herein, near-infrared (NIR) light-responded vancomycin-doped prussian blue nanoparticles (PB-VANNPs) with high efficient photothermal conversion was synthesized for binding, dual-mode portable detection, and elimination of bacteria. The PB-VANNPs can bind to the surface of Gram-positive bacteria such as Staphylococcus aureus (S. aureus), forming complex of PB-VANNPs/S. aureus. After being centrifugated, the suspension solution of PB-VANNPs can stimulate perfluorohexane (PFH) to rapidly release oxygen (O2) under NIR irradiation. Thus, the bacteria can be sensitively detected with portable pressure meter as signal reader, reporting a limit of detection (LOD) of 1.0 CFU mL-1. On the other side, the sediment of PB-VANNPs/S. aureus can be detected via thermal camera, reporting a LOD of 1.0 CFU mL-1. Interestingly, the bacteria can be effectively inactivated with the local temperature elevation during temperature-based detection. The antibacterial efficiency reaches as high as 99.8%. The developed multifunctional nanoplatform not only provides a straightforward "mix-then-test" way for portable detection of bacteria with high sensitivity, also realizes high efficiency elimination of bacteria simultaneously. The developed strategy was further applied for promoting wound healing of bacteria-infected mice.

Biosensors Based on Mechanical and Electrical Detection Techniques

Biosensors are powerful analytical tools for biology and biomedicine, with applications ranging from drug discovery to medical diagnostics, food safety, and agricultural and environmental monitoring. Typically, biological recognition receptors, such as enzymes, antibodies, and nucleic acids, are immobilized on a surface, and used to interact with one or more specific analytes to produce a physical or chemical change, which can be captured and converted to an optical or electrical signal by a transducer. However, many existing biosensing methods rely on chemical, electrochemical and optical methods of identification and detection of specific targets, and are often: complex, expensive, time consuming, suffer from a lack of portability, or may require centralised testing by qualified personnel. Given the general dependence of most optical and electrochemical techniques on labelling molecules, this review will instead focus on mechanical and electrical detection techniques that can provide information on a broad range of species without the requirement of labelling. These techniques are often able to provide data in real time, with good temporal sensitivity. This review will cover the advances in the development of mechanical and electrical biosensors, highlighting the challenges and opportunities therein.