Cantilever biosensors

Molecular-electromechanical system for unamplified detection of trace analytes in biofluids

Biological research and diagnostic applications normally require analysis of trace analytes in biofluids. Although considerable advancements have been made in developing precise molecular assays, the trade-off between sensitivity and ability to resist non-specific adsorption remains a challenge. Here, we describe the implementation of a testing platform based on a molecular-electromechanical system (MolEMS) immobilized on graphene field-effect transistors. A MolEMS is a self-assembled DNA nanostructure, containing a stiff tetrahedral base and a flexible single-stranded DNA cantilever. Electromechanical actuation of the cantilever modulates sensing events close to the transistor channel, improving signal-transduction efficiency, while the stiff base prevents non-specific adsorption of background molecules present in biofluids. A MolEMS realizes unamplified detection of proteins, ions, small molecules and nucleic acids within minutes and has a limit of detection of several copies in 100 μl of testing solution, offering an assay methodology with wide-ranging applications. In this protocol, we provide step-by-step procedures for MolEMS design and assemblage, sensor manufacture and operation of a MolEMS in several applications. We also describe adaptations to construct a portable detection platform. It takes ~18 h to construct the device and ~4 min to finish the testing from sample addition to result.

Aptamers Targeting Membrane Proteins for Sensor and Diagnostic Applications

Many biological processes (physiological or pathological) are relevant to membrane proteins (MPs), which account for almost 30% of the total of human proteins. As such, MPs can serve as predictive molecular biomarkers for disease diagnosis and prognosis. Indeed, cell surface MPs are an important class of attractive targets of the currently prescribed therapeutic drugs and diagnostic molecules used in disease detection. The oligonucleotides known as aptamers can be selected against a particular target with high affinity and selectivity by iterative rounds of in vitro library evolution, known as Systematic Evolution of Ligands by EXponential Enrichment (SELEX). As an alternative to antibodies, aptamers offer unique features like thermal stability, low-cost, reuse, ease of chemical modification, and compatibility with various detection techniques. Particularly, immobilized-aptamer sensing platforms have been under investigation for diagnostics and have demonstrated significant value compared to other analytical techniques. These "aptasensors" can be classified into several types based on their working principle, which are commonly electrochemical, optical, or mass-sensitive. In this review, we review the studies on aptamer-based MP-sensing technologies for diagnostic applications and have included new methodological variations undertaken in recent years.

Advanced implications of nanotechnology in disease control and environmental perspectives

Nanotechnology encompasses a wide range of devices derived from biology, engineering, chemistry, and physics, and this scientific field is composed of great collaboration among researchers from several fields. It has diverse implications notably smart sensing technologies, effective disease diagnosis, and sometimes used in treatment. In medical science, the implications of nanotechnology include the development of elements and devices that interact with the body at subcellular (i.e., molecular) levels exhibiting high sensitivity and specificity. There is a plethora of new chances for medical science and disease treatment to be discovered and exploited in the rapidly developing field of nanotechnology. In different sectors, nanomaterials are used just because of their special characteristics. Their large surface area of them enables higher reactivity with greater efficiency. Furthermore, special surface chemistry is displayed by nanomaterials which compare to conventional materials and facilitate the nanomaterials to decrease pollutants efficiently. Recently, nanomaterials are used in some countries to reduce the levels of contaminants in water, air, and soil. Moreover, nanomaterials are used in the cosmetics and medical industry, and it develops the drug discovery (DD) system. Among a huge number of nanomaterials, Cu, Ag, TiO₂, ZnO, Fe₃O₄, and carbon nanotubes (CNTs) are extensively used in different industries for various purposes. This extensive review study has introduced the major scientific and technical features of nanotechnology, as well as some possible clinical applications and positive feedback in environmental waste management and drug delivery systems.

Recent Advances in Nanotechnology for the Management of Klebsiella pneumoniae-Related Infections

Klebsiella pneumoniae is an important human pathogen that causes diseases such as urinary tract infections, pneumonia, bloodstream infections, bacteremia, and sepsis. The rise of multidrug-resistant strains has severely limited the available treatments for K. pneumoniae infections. On the other hand, K. pneumoniae activity (and related infections) urgently requires improved management strategies. A growing number of medical applications are using nanotechnology, which uses materials with atomic or molecular dimensions, to diagnose, eliminate, or reduce the activity of different infections. In this review, we start with the traditional treatment and detection method for K. pneumoniae and then concentrate on selected studies (2015-2022) that investigated the application of nanoparticles separately and in combination with other techniques against K. pneumoniae.

Point-of-Care Diagnostics for Farm Animal Diseases: From Biosensors to Integrated Lab-on-Chip Devices

Zoonoses and animal diseases threaten human health and livestock biosecurity and productivity. Currently, laboratory confirmation of animal disease outbreaks requires centralized laboratories and trained personnel; it is expensive and time-consuming, and it often does not coincide with the onset or progress of diseases. Point-of-care (POC) diagnostics are rapid, simple, and cost-effective devices and tests, that can be directly applied on field for the detection of animal pathogens. The development of POC diagnostics for use in human medicine has displayed remarkable progress. Nevertheless, animal POC testing has not yet unfolded its full potential. POC devices and tests for animal diseases face many challenges, such as insufficient validation, simplicity, and portability. Emerging technologies and advanced materials are expected to overcome some of these challenges and could popularize animal POC testing. This review aims to: (i) present the main concepts and formats of POC devices and tests, such as lateral flow assays and lab-on-chip devices; (ii) summarize the mode of operation and recent advances in biosensor and POC devices for the detection of farm animal diseases; (iii) present some of the regulatory aspects of POC commercialization in the EU, USA, and Japan; and (iv) summarize the challenges and future perspectives of animal POC testing.

Biomarkers and Detection Platforms for Human Health and Performance Monitoring: A Review

Human health and performance monitoring (HHPM) is imperative to provide information necessary for protecting, sustaining, evaluating, and improving personnel in various occupational sectors, such as industry, academy, sports, recreation, and military. While various commercially wearable sensors are on the market with their capability of "quantitative assessments" on human health, physical, and psychological states, their sensing is mostly based on physical traits, and thus lacks precision in HHPM. Minimally or noninvasive biomarkers detectable from the human body, such as body fluid (e.g., sweat, tear, urine, and interstitial fluid), exhaled breath, and skin surface, can provide abundant additional information to the HHPM. Detecting these biomarkers with novel or existing sensor technologies is emerging as critical human monitoring research. This review provides a broad perspective on the state of the art biosensor technologies for HHPM, including the list of biomarkers and their physiochemical/physical characteristics, fundamental sensing principles, and high-performance sensing transducers. Further, this paper expands to the additional scope on the key technical challenges in applying the current HHPM system to the real field.

Surface-Stress Induced Embrittlement of Metals

Environment-assisted fracture phenomena in metals are usually associated with surface energy reduction due to an adsorbed film. Here we demonstrate a unique embrittlement effect in Al that is instead mediated by surface stress, induced by an adsorbed organic monolayer. Atomistic simulations show that the adsorbate carbon-chain length l_c controls the surface stress via van der Waals forces, being compressive for l_c < 8 and tensile otherwise. For l_c > 8, we demonstrate experimentally that the nanoscale film causes a ductile-to-brittle transition on the macroscale. Concomitant with this transition is a nearly 85% reduction in deformation forces. Additional simulations reveal that the microscopic mechanism for the embrittlement is via suppression of dislocation emission at incipient crack-tips. In addition to challenging long-held views on environment-assisted fracture, our findings pertaining to surface-stress induced embrittlement suggest profitable utility in manufacturing processes such as machining and comminution.

Nanomotion Detection-Based Rapid Antibiotic Susceptibility Testing

Rapid antibiotic susceptibility testing (AST) could play a major role in fighting multidrug-resistant bacteria. Recently, it was discovered that all living organisms oscillate in the range of nanometers and that these oscillations, referred to as nanomotion, stop as soon the organism dies. This finding led to the development of rapid AST techniques based on the monitoring of these oscillations upon exposure to antibiotics. In this review, we explain the working principle of this novel technique, compare the method with current ASTs, explore its application and give some advice about its implementation. As an illustrative example, we present the application of the technique to the slowly growing and pathogenic Bordetella pertussis bacteria.

Label-free attomolar protein detection using a MEMS optical interferometric surface-stress immunosensor with a freestanding PMMA/parylene-C nanosheet

We demonstrated an optical interferometer-based surface-stress immunosensor using freestanding polymethyl methacrylate (PMMA)/parylene-C nanosheet with high sensitivity for detection of biomolecules. PMMA/parylene-C nanosheets were transferred onto a silicon substrate with microcavities to fabricate freestanding submicron-thick membrane with a sealed cavity structure. The adhesive force between the transferred parylene-C and binder parylene-C layer was measured to be 1.06-2.4 N/10 mm by tape test. Evading Debye shielding, these nanomechanical sensors allow detection of the adsorption on the membrane surface through changes in surface stress transduced by the electric charge. We optimized the density of receptors and mode of immobilization for high sensitivity. To evaluate the selectivity of the sensor, membrane deflections induced by various proteins were measured and the spectral shifts showed high selectivity only for the target antigen. The minimum limit of detection (LOD) of the sensor for human serum albumin antigen was 0.1-1 fg/mL (1.5-15 aM), which was 20,000 times lower than that of the conventional micro-cantilever sensor.

Organic monolayers disrupt plastic flow in metals

Adsorbed films often influence mechanical behavior of surfaces, leading to well-known mechanochemical phenomena such as liquid metal embrittlement and environment-assisted cracking. Here, we demonstrate a mechanochemical phenomenon wherein adsorbed long-chain organic monolayers disrupt large-strain plastic deformation in metals. Using high-speed in situ imaging and post facto analysis, we show that the monolayers induce a ductile-to-brittle transition. Sinuous flow, characteristic of ductile metals, gives way to quasi-periodic fracture, typical of brittle materials, with 85% reduction in deformation forces. By independently varying surface energy and molecule chain length via molecular self-assembly, we argue that this "embrittlement" is driven by adsorbate-induced surface stress, as against surface energy reduction. Our observations, backed by modeling and molecular simulations, could provide a basis for explaining diverse mechanochemical phenomena in solids. The results also have implications for manufacturing processes such as machining and comminution, and wear.

Pressure Sensitive Adhesive Tape: A Versatile Material Platform for Optical Sensors

Pressure sensitive adhesive (PSA) tapes are a versatile, safe and easy-to-use solution for fastening, sealing, masking, or joining. They are widely employed in daily life, from domestic use to industrial applications in sectors such as construction and the automotive industry. In recent years, PSA tapes have found a place in the field of micro- and nanotechnology, particularly in contact transfer techniques where they can be used as either sacrificial layers or flexible substrates. As a consequence, various optical sensing configurations based on PSA tapes have been developed. In this paper, recent achievements related to the use of PSA tapes as functional and integral parts of optical sensors are reviewed. These include refractive index sensors, optomechanical sensors and vapor sensors.

Review of Gravimetric Sensing of Volatile Organic Compounds

Volatile organic compounds (VOCs) are pervasive in the environment. Since the early 1980s, substantial work has examined the detection of these materials, as they can indicate environmental changes that can affect human health. VOCs and similar compounds present a very specific sensing problem in that they are not reactive and often nonpolar, so it is difficult to find materials that selectively bind or adsorb them. A number of techniques are applied to vapor sensing. High resolution molecular separation approaches such as gas chromatography and mass spectrometry are well-characterized and offer high sensitivity, but are difficult to implement in portable, real-time monitors, whereas approaches such as chemiresistors are promising, but still in development. Gravimetric approaches, in which the mass of an adsorbed vapor is directly measured, have several potential advantages over other techniques but have so far lagged behind other approaches in performance and market penetration. This review aims to offer a comprehensive background on gravimetric sensing including underlying resonators and sensitizers, as well as a picture of applications and commercialization in the field.

Critical assessment of relevant methods in the field of biosensors with direct optical detection based on fibers and waveguides using plasmonic, resonance, and interference effects

Direct optical detection has proven to be a highly interesting tool in biomolecular interaction analysis to be used in drug discovery, ligand/receptor interactions, environmental analysis, clinical diagnostics, screening of large data volumes in immunology, cancer therapy, or personalized medicine. In this review, the fundamental optical principles and applications are reviewed. Devices are based on concepts such as refractometry, evanescent field, waveguides modes, reflectometry, resonance and/or interference. They are realized in ring resonators; prism couplers; surface plasmon resonance; resonant mirror; Bragg grating; grating couplers; photonic crystals, Mach-Zehnder, Young, Hartman interferometers; backscattering; ellipsometry; or reflectance interferometry. The physical theories of various optical principles have already been reviewed in detail elsewhere and are therefore only cited. This review provides an overall survey on the application of these methods in direct optical biosensing. The "historical" development of the main principles is given to understand the various, and sometimes only slightly modified variations published as "new" methods or the use of a new acronym and commercialization by different companies. Improvement of optics is only one way to increase the quality of biosensors. Additional essential aspects are the surface modification of transducers, immobilization strategies, selection of recognition elements, the influence of non-specific interaction, selectivity, and sensitivity. Furthermore, papers use for reporting minimal amounts of detectable analyte terms such as value of mass, moles, grams, or mol/L which are difficult to compare. Both these essential aspects (i.e., biochemistry and the presentation of LOD values) can be discussed only in brief (but references are provided) in order to prevent the paper from becoming too long. The review will concentrate on a comparison of the optical methods, their application, and the resulting bioanalytical quality.

A suspended graphene-based optical interferometric surface stress sensor for selective biomolecular detection

Graphene-based sensors are of great interest in research due to their high specific surface area and high electron mobility that make them suitable for numerous advanced applications. In this paper, selective molecular detection using an antigen-antibody reaction on suspended graphene with a cavity-sealing structure was demonstrated. The suspended graphene sealed nanocavities in a pre-patterned Si substrate, which increased robustness and allowed the use of wet chemical processes for surface functionalization of the suspended graphene to achieve selective molecular binding. The selectivity was evaluated by nanomechanical deflection induced by molecular adsorption on the suspended graphene, resulting in spectral shifts in the optical interference between the suspended graphene and Si substrate. The chemically functionalized suspended graphene enables the analysis of intermolecular interactions and molecular kinetics by colorimetry using optical interference.

Sensitive detection of cardiac troponin-I protein using fabricated piezoresistive microcantilevers by a novel method of asymmetric biofunctionalization

Microcantilever-based sensor platform has attracted a lot of attention over the time in detection of a variety of molecules due to their miniaturized dimensions. Sensitivity enhancement is an important aspect of such sensors, especially when used for point-of-care diagnostic purpose. However, the major concern while operating these sensors in deflection mode is their sensitivity which mainly relies on selective chemical modification protocols employed on these sensor surfaces. One of the ways of getting better sensitivity is through asymmetric (one side) biofunctionalization of the sensor surface. In the presented work here, we have demonstrated a novel approach of asymmetric biofunctionalization of proteins in overall sensitivity enhancement of piezoresistive silicon nitride-oxide microcantilever sensor platform inside a flow chamber. Herein, using our developed surface chemistry, asymmetrically biofunctionalized microcantilevers first exhibited a greater electrical response in terms of piezoresistance change than their symmetric counterpart in the detection of human immunoglobulins (HIgGs) protein. Finally, these microcantilevers were employed to exhibit the enhanced sensitivity towards the detection of a crucial cardiac marker protein, i.e. Troponin-I (cTnI) down to 250 ng ml^-1 using asymmetric biofunctionalization process. This study shows that the developed asymmetric biofunctionalization methodology may be used as a general protocol to detect other important biomarkers of clinical applications with improved sensitivity.

Silicon Microcantilever Sensors to Detect the Reversible Conformational Change of a Molecular Switch, Spiropyan

The high sensitivity of silicon microcantilever sensors has expanded their use in areas ranging from gas sensing to bio-medical applications. Photochromic molecules also represent promising candidates for a large variety of sensing applications. In this work, the operating principles of these two sensing methods are combined in order to detect the reversible conformational change of a molecular switch, spiropyran. Thus, arrays of silicon microcantilever sensors were functionalized with spiropyran on the gold covered side and used as test microcantilevers. The microcantilever deflection response was observed, in five sequential cycles, as the transition from the spiropyran (SP) (CLOSED) to the merocyanine (MC) (OPEN) state and vice-versa when induced by UV and white light LED sources, respectively, proving the reversibility capabilities of this type of sensor. The microcantilever deflection direction was observed to be in one direction when changing to the MC state and in the opposite direction when changing back to the SP state. A tensile stress was induced in the microcantilever when the SP to MC transition took place, while a compressive stress was observed for the reverse transition. These different type of stresses are believed to be related to the spatial conformational changes induced in the photochromic molecule upon photo-isomerisation.

Piezoelectric BioMEMS Cantilever for Measurement of Muscle Contraction and for Actuation of Mechanosensitive Cells

A piezoelectric biomedical microelectromechanical system (bioMEMS) cantilever device was designed and fabricated to act as either a sensing element for muscle tissue contraction or as an actuator to apply mechanical force to cells. The sensing ability of the piezoelectric cantilevers was shown by monitoring the electrical signal generated from the piezoelectric aluminum nitride in response to the contraction of iPSC-derived cardiomyocytes cultured on the piezoelectric cantilevers. Actuation was demonstrated by applying electrical pulses to the piezoelectric cantilever and observing bending via an optical detection method. This piezoelectric cantilever device was designed to be incorporated into body-on-a-chip systems.

Nanomaterials based optical and electrochemical sensing of histamine: Progress and perspectives

Histamine is known to be a principal causative agent associated with marine food poisoning outbreaks worldwide, which is typically formed in the contaminated food by decarboxylation of histidine by bacterial histidine decarboxylase. Upon quantification of histamine in different food products, one can comment on the quality of the food and use it as an indicator of the good manufacturing practices and the state of preservation. The United States Food and Drug Administration (FDA) has established 50 ppm (50 mg/kg) of histamine as the chemical index for fish spoilage. Consumption of foods containing histamine higher than the permissible limit can cause serious health issues. Several methods have been developed for the determination of histamine in a variety of food products. The conventional methods for histamine detection such as thin layer chromatography, capillary zone electrophoresis, gas chromatography, colorimetry, fluorimetry, ion mobility spectrometry, high-performance liquid chromatography, and enzyme-linked immunosorbent assay (ELISA), are being used for sensitive and selective detection of histamine. However, there are a number of disadvantages associated with the conventional techniques, such as multi-step sample processing and requirement of expensive sophisticated instruments, which restrict their applications at laboratory level only. In order to address the limitations associated with the traditional methods, new approaches have been developed by various research groups. Current advances in nanomaterial-based sensing of histamine in different food products have shown significant measurement accuracy due to their high sensitivity, specificity, field deployability, cost and ease of operation. In this review, we have discussed the development of nanomaterials-based histamine sensing assays/strategies where the detection is based on optical (fluorescence, surface enhanced Raman spectroscopy (SERS), localized surface plasmon resonance) and electrochemical (impedimetric, voltammetry, potentiometric, etc.). Further, the advantages, disadvantages and future scope of the nanomaterials-based histamine sensor research are highlighted.

Anomalous interfacial stress generation during sodium intercalation/extraction in MoS2 thin-film anodes

Although the generation of mechanical stress in the anode material is suggested as a possible reason for electrode degradation and fading of storage capacity in batteries, only limited knowledge of the electrode stress and its evolution is available at present. Here, we show real-time monitoring of the interfacial stress of a few-layer MoS₂ system under the sodiation/desodiation process using microcantilever electrodes. During the first sodiation with a voltage plateau of 1.0 to 0.85 V, the MoS₂ exhibits a compressive stress (2.1 Nm^-1), which is substantially smaller than that measured (9.8 Nm^-1) during subsequent plateaus at 0.85 to 0.4 V due to the differential volume expansion of the MoS₂ film. The conversion reaction to Mo below 0.1 V generates an anomalous compressive stress of 43 Nm^-1 with detrimental effects. These results also suggest the existence of a separate discharge stage between 0.6 and 0.1 V, where the generated stress is only approximately one-third of that observed below 0.1 V. This approach can be adapted to help resolve the localized stress in a wide range of electrode materials, to gain additional insights into mechanical effects of charge storage, and for long-lifetime battery design.

Biochemical sensing with macrocyclic receptors

Preventive healthcare asks for the development of cheap, precise and non-invasive sensor devices for the early detection of diseases and continuous population screening. The actual techniques used for diagnosis, e.g. MRI and PET, or for biochemical marker sensing, e.g. immunoassays, are not suitable for continuous monitoring since they are expensive and prone to false positive responses. Synthetic supramolecular receptors offer new opportunities for the creation of specific, selective and cheap sensor devices for biological sensing of specific target molecules in complex mixtures of organic substances. The fundamental challenges faced in developing such devices are the precise transfer of the molecular recognition events at the solid-liquid interface and its transduction into a readable signal. In this review we present the progress made so far in turning synthetic macrocyclic hosts, namely cyclodextrins, calixarenes, cucurbiturils and cavitands, into effective biochemical sensors and the strategies utilized to solve the above mentioned issues. The performances of the developed sensing devices based on these receptors in detecting specific biological molecules, drugs and proteins are critically discussed.

Flow force augmented 3D suspended polymeric microfluidic (SPMF3 ) platform

Detection and study of bioelements using microfluidic systems has been of great interest in the biodiagnostics field. Microcantilevers are the most used systems in biodetection due to their implementation simplicity which have been used for a wide variety of applications ranging from cellular to molecular diagnosis. However, increasing further the sensitivity of the microcantilever systems have a great effect on the cantilever based sensing for chemical and bio applications. In order to improve further the performance of microcantilevers, a flow force augmented 3D suspended microchannel is proposed using which microparticles can be conveyed through a microchannel inside the microcantilever to the detection area. This innovative microchannel design addresses the low sensitivity issue by increasing its sensitivity up to 5 times than the earlier reported similar microsystems. Moreover, fabricating this microsystem out of Polydimethylsiloxane (PDMS) would eliminate external exciter dependency in many detection applications such as biodiagnostics. In this study, the designed microsystem has been analyzed theoretically, simulated and tested. Moreover, the microsystem has been fabricated and tested under different conditions, the results of which have been compared with simulation results. Finally, its innovative fabrication process and issues are reported and discussed.

Micromechanical Redox Actuation by Self-Assembled Monolayers of Ferrocenylalkanethiolates: Evens Push More Than Odds

Microcantilever transducers can be valuable tools for the investigation of physicochemical processes in organized molecular films. Gold-coated cantilevers are used here to investigate the electrochemomechanics of redox-active self-assembled monolayers (SAMs) of ferrocenylalkanethiolates (Fc(CH₂) _nS) of different alkyl chain lengths. A significant odd-even effect is observed in the surface stress and cantilever movement generated by the oxidation of the SAM-confined ferrocenes as the number of methylene units n in the SAM backbone is varied. We demonstrate that stronger alkyl chain-chain interactions are at the origin of the larger surface stresses generated by SAMs with an even versus odd n. The findings highlight the impact of subtle structural effects and weak van der Waals interactions on the mechanical actuation produced by redox reactions in self-assembled systems.

Microfluidic Line-Free Mass Sensor Based on an Antibody-Modified Mechanical Resonator

This research proposes a mass sensor based on mechanical resonance that is free from power supply lines (line-free) and incorporates both microfluidic mechanisms and label-free techniques to improve its sensitivity and reusability. The microfluidic line-free mass sensor comprises a disk-shaped mechanical resonator, a separate piezoelectric element used to excite vibrations in the resonator, and a microfluidic mechanism. Electrical power is used to actuate the piezoelectric element, leaving the resonator free from power lines. The microfluidic mechanism allows for rapid, repeat washings to remove impurities from a sample. The microfluidic line-free mass sensor is designed as a label-free sensor to enable high-throughput by modifying and dissociating an antibody on the resonator. The resonator was fabricated by photolithography and the diameter and thickness were 4 mm and 0.5 mm, respectively. The line-free mass sensor enabled a high Q-factor and resonance frequency of 7748 MHz and 1.402 MHz, respectively, to be achieved even in liquids, facilitating the analysis of human salivary cortisol. The line-free mass sensor could be used for repeated measurements with the microfluidic mechanism, and the resonator could be fully washed out. It was concluded that the microfluidic line-free mass sensor was suitable to analyze the concentration of a salivary hormone, cortisol, in human saliva samples, and that it provided high-throughput suitable for point-of-care testing.

Polymeric Flexible Immunosensor Based on Piezoresistive Micro-Cantilever with PEDOT/PSS Conductive Layer

In this paper, a fully polymeric micro-cantilever with the surface passivation layer of parylene-C and the strain resistor of poly(3,4-ethylenedioxythiophene)/poly (styrene sulfonate) (PEDOT/PSS) was proposed and demonstrated for immunoassays. By optimizing the design and fabrication of the polymeric micro-cantilever, a square resistance of 220 Ω/□ for PEDOT/PSS conductive layer have been obtained. The experimental spring constant and the deflection sensitivity were measured to be 0.017 N/m and 8.59 × 10-7 nm-1, respectively. The biological sensing performances of polymeric micro-cantilever were investigated by the immunoassay for human immunoglobulin G (IgG). The immunosensor was experimentally demonstrated to have a linear behavior for the detection of IgG within the concentrations of 10~100 ng/mL with a limit of detection (LOD) of 10 ng/mL. The experimental results indicate that the proposed polymeric flexible conductive layer-based sensors are capable of detecting trace biological substances.

Identifying DNA mismatches at single-nucleotide resolution by probing individual surface potentials of DNA-capped nanoparticles

Here, we demonstrate a powerful method to discriminate DNA mismatches at single-nucleotide resolution from 0 to 5 mismatches (χ₀ to χ₅) using Kelvin probe force microscopy (KPFM). Using our previously developed method, we quantified the surface potentials (SPs) of individual DNA-capped nanoparticles (DCNPs, ∼100 nm). On each DCNP, DNA hybridization occurs between ∼2200 immobilized probe DNA (pDNA) and target DNA with mismatches (tDNA, ∼80 nM). Thus, each DCNP used in the bioassay (each pDNA-tDNA interaction) corresponds to a single ensemble in which a large number of pDNA-tDNA interactions take place. Moreover, one KPFM image can scan at least dozens of ensembles, which allows statistical analysis (i.e., an ensemble average) of many bioassay cases (ensembles) under the same conditions. We found that as the χ_n increased from χ₀ to χ₅ in the tDNA, the average SP of dozens of ensembles (DCNPs) was attenuated owing to fewer hybridization events between the pDNA and the tDNA. Remarkably, the SP attenuation vs. the χ_n showed an inverse-linear correlation, albeit the equilibrium constant for DNA hybridization exponentially decreased asymptotically as the χ_n increased. In addition, we observed a cascade reaction at a 100-fold lower concentration of tDNA (∼0.8 nM); the average SP of DCNPs exhibited no significant decrease but rather split into two separate states (no-hybridization vs. full-hybridization). Compared to complementary tDNA (i.e., χ₀), the ratio of no-hybridization/full-hybridization within a given set of DCNPs became ∼1.6 times higher in the presence of tDNA with single mismatches (i.e., χ₁). The results imply that our method opens new avenues not only in the research on the DNA hybridization mechanism in the presence of DNA mismatches but also in the development of a robust technology for DNA mismatch detection.

Thermomechanical analysis of picograms of polymers using a suspended microchannel cantilever

Suspended microchannel cantilever offers high sensitivity of detecting various thermal transitions of picogram amount of polymers which is showed by the schematic and the SEM of the actual device and the data obtained from it.

Ultrasensitive, Label Free, Chemiresistive Nanobiosensor Using Multiwalled Carbon Nanotubes Embedded Electrospun SU-8 Nanofibers

This paper reports the synthesis and fabrication of aligned electrospun nanofibers derived out of multiwalled carbon nanotubes (MWCNTs) embedded SU-8 photoresist, which are targeted towards ultrasensitive biosensor applications. The ultrasensitivity (detection in the range of fg/mL) and the specificity of these biosensors were achieved by complementing the inherent advantages of MWCNTs such as high surface to volume ratio and excellent electrical and transduction properties with the ease of surface functionalization of SU-8. The electrospinning process was optimized to precisely align nanofibers in between two electrodes of a copper microelectrode array. MWCNTs not only enhance the conductivity of SU-8 nanofibers but also act as transduction elements. In this paper, MWCNTs were embedded way beyond the percolation threshold and the optimum percentage loading of MWCNTs for maximizing the conductivity of nanofibers was figured out experimentally. As a proof of concept, the detection of myoglobin, an important biomarker for on-set of Acute Myocardial Infection (AMI) has been demonstrated by functionalizing the nanofibers with anti-myoglobin antibodies and carrying out detection using a chemiresistive method. This simple and robust device yielded a detection limit of 6 fg/mL.

Extended-gate field-effect transistor packed in micro channel for glucose, urea and protein biomarker detection

This study developed a packaging method to integrate the extended-gate field-effect transistor (EGFET) into a microfluidic chip as a biological sensor. In addition, we present two immobilization approaches for the bio-recognition that are appropriate to this chip, allowing it to measure the concentrations of hydrogen ions, glucose, urea, and specific proteins in a solution. Alginate-calcium microcubes were used to embed the enzymes and magnetic powder (enzyme carrier). When the sensing chip needs the enzyme for the catalytic reaction, the alginate microcubes containing the corresponding enzymes enter through the flow channel and are immobilized on the EGFET surface with an external magnet. High sensing performance of the chip is achieved, with 37.45 mV/mM for measuring hydrogen ions at pH 6-8 with a linearity of 0.9939, 7.00 mV/mM for measuring glucose with a linearity of 0.9962, and 8.01 mV/mM for measuring urea with a linearity of 0.9809. In addition, based on the principle of the immunoassay, the magnetic beads with the specific antibody were used to capture the target protein in the sample. Then, negatively charged DNA fragments bound to a secondary antibody were used to amplify the signal for EGFET measurement. The magnetic beads with completed immune response bonding were then fixed on the surface of the sensor by an external magnetic field. Therefore, the measured object can directly contact the sensor surface, and quantitative detection of the protein concentration can be achieved. Apolipoprotein A1 (APOA1) was detected as a target protein, with a minimum detection limit of approximately 12.5 ng/mL.

Asymmetric immobilization of antibodies on a piezo-resistive micro-cantilever surface

For cantilever-based MEMS sensors, selective chemical modification of the sensing surface is used for the detection of chemical and biological analytes.

Proposition of a Silica Nanoparticle-Enhanced Hybrid Spin-Microcantilever Sensor Using Nonlinear Optics for Detection of DNA in Liquid

We theoretically propose a method based on the combination of a nonlinear optical mass sensor using a hybrid spin-microcantilever and the nanoparticle-enhanced technique, to detect and monitor DNA mutations. The technique theoretically allows the mass of external particles (ssDNA) landing on the surface of a hybrid spin-microcantilever to be detected directly and accurately at 300 K with a mass responsivity 0.137 Hz/ag in situ in liquid. Moreover, combined with the nanoparticle-enhanced technique, even only one base pair mutation in the target DNA sequence can be identified in real time accurately, and the DNA hybridization reactions can be monitored quantitatively. Furthermore, in situ detection in liquid and measurement of the proposed nonlinear optical spin resonance spectra will minimize the experimental errors.

Characterization of a gold coated cantilever surface for biosensing applications

Cantilever based sensors are a promising tool for a very diverse spectrum of biological sensors. They have been used for the detection of proteins, DNA, antigens, bacteria viruses and many other biologically relevant targets. Although cantilever sensing has been described for over 20 years, there are still no viable commercial cantilever-based sensing products on the market. Several reasons can be found for this - a lack of detailed understanding of the origin of signals being an important one. As a consequence application-relevant issues such as shelf life and robust protocols distinguishing targets from false responses have received very little attention. Here, we will discuss a cantilever sensing platform combined with an electrochemical system. The detected surface stress signal is modulated by applying a square wave potential to a gold coated cantilever. The square wave potential induces adsorption and desorption onto the gold electrode surface as well as possible structural changes of the target and probe molecules on the cantilever surface resulting in a measurable surface stress change. What sets this approach apart from regular cantilever sensing is that the quantification and identification of observed signals due to target-probe interactions are not only a function of stress value (i.e. amplitude), but also of the temporal evolution of the stress response as a function of the rate and magnitude of the applied potential change, and the limits of the potential change. This paper will discuss three issues that play an important role in future successful applications of cantilever-based sensing. First, we will discuss what is required to achieve a large surface stress signal to improve sensitivity. Second, a mechanism to achieve an optimal probe density is described that improves the signal-to-noise ratio and response times of the sensor. Lastly, lifetime and long term measurements are discussed.

Micro- and nano-structure based oligonucleotide sensors

This paper presents a review of micro- and nano-structure based oligonucleotide detection and quantification techniques. The characteristics of such devices make them very attractive for Point-of-Care or On-Site-Testing biosensing applications. Their small scale means that they can be robust and portable, their compatibility with modern CMOS electronics means that they can easily be incorporated into hand-held devices and their suitability for mass production means that, out of the different approaches to oligonucleotide detection, they are the most suitable for commercialisation. This review discusses the advantages of micro- and nano-structure based sensors and covers the various oligonucleotide detection techniques that have been developed to date. These include: Bulk Acoustic Wave and Surface Acoustic Wave devices, micro- and nano-cantilever sensors, gene Field Effect Transistors, and nanowire and nanopore based sensors. Oligonucleotide immobilisation techniques are also discussed.

Synthetic sialylglycopolymer receptor for virus detection using cantilever-based sensors

We describe the rapid, label-free detection of Influenza A viruses using a cantilever transducer modified with a synthetic sialylglycopolymer receptor layer.

Biosensors for hepatitis B virus detection

A biosensor is an analytical device used for the detection of analytes, which combines a biological component with a physicochemical detector. Recently, an increasing number of biosensors have been used in clinical research, for example, the blood glucose biosensor. This review focuses on the current state of biosensor research with respect to efficient, specific and rapid detection of hepatitis B virus (HBV). The biosensors developed based on different techniques, including optical methods (e.g., surface plasmon resonance), acoustic wave technologies (e.g., quartz crystal microbalance), electrochemistry (amperometry, voltammetry and impedance) and novel nanotechnology, are also discussed.

Theoretical modeling and experimental validation of surface stress in thrombin aptasensor

Adsorption of target molecules on the immobilized microcantilever surface produced beam displacement due to the differential surface stress generated between the immobilized and non-immobilized surface. Surface stress is caused by the intermolecular forces between the molecules. Van der Waals, electrostatic forces, hydrogen bonding, hydrophobic effect and steric hindrance are some of the intermolecular forces involved. A theoretical framework describing the adsorption-induced microcantilever displacement is derived in this paper. Experimental displacement of thrombin aptamer-thrombin interactions was carried out. The relation between the electrostatic interactions involved between adsorbates (thrombin) as well as adsorbates and substrates (thrombin aptamer) and the microcantilever beam displacement utilizing the proposed mathematical model was quantified and compared to the experimental value. This exercise is important to aid the designers in microcantilever sensing performance optimization.

Carbohydrate coating reduces adhesion of biofilm-forming Bacillus subtilis to gold surfaces

The growth of bacterial biofilms in pipes and food tanks causes severe problems in industry. Biofilms growing on medical implants or catheters are of great concern, as they can cause serious infections and decrease the functionality of the medical device. The prevention of bacterial adhesion--the first step in colonization and biofilm formation--is therefore very important. Current research comprises alterations in surface properties, the prevention of adhesin biosynthesis, inhibition with receptor analogs, or the development of anti-adhesive vaccines. We present a new approach that allows us to study bacterial adhesion with high sensitivity in real-time while testing several different surfaces in parallel. Using the cantilever-array technique we demonstrate that coating of gold surfaces with mono- or disaccharides results in a reduction of the bacterial adhesion of the biofilm-forming bacterium Bacillus subtilis NCIB 3610 to these gold surfaces. This reduction in bacterial adhesion is independent of the studied carbohydrate. Using several mutant strains, we investigate the underlying molecular interactions, and our results suggest that adhesion to gold surfaces is mediated by thiol groups present in proteins of the bacterial cell membrane or biofilm matrix proteins expressed at low levels by the wild-type strain. Furthermore, our data indicate that the adhesion of B. subtilis NCIB 3610 to carbohydrate-coated gold surfaces is facilitated by interactions between carbohydrates installed on the cantilever gold surface and an exopolysaccharide expressed by this strain. Understanding general and specific contributions of molecular interactions mediating bacterial adhesion will enable its prevention in the future.

Modeling a Nanocantilever-Based Biosensor Using a Stochastically Perturbed Harmonic Oscillator

Nanoscale biosensors are devices designed to detect analytes by combining biological components and physicochemical detectors. A well-known design of these sensors involves the implementation of nanocantilevers. These microscopic diving boards are coated with binding probes that have an affinity for a particular amino acid, enzyme or protein in living organisms. When these probes attract target particles, such as biomolecules, the binding of these particles changes the vibrating frequency of the cantilever. This process is random in nature and produces fluctuations in the frequency and damping of the cantilever. In this paper, we studied the effect of these fluctuations using a stochastically perturbed, classical harmonic oscillator.

Structural modifications of gold thin films produced by thiol-derivatized single-stranded DNA immobilization

Recent experiments have reported an opposite sign of the differential surface stress produced on gold-coated cantilevers by a thiol-derivatized single-stranded DNA (SH-DNA) immobilization process. The sign of the surface stress depends on the method used to evaporate the gold thin film, being compressive (negative) or tensile (positive) for e-beam or resistively deposited gold, respectively. This study investigates the origin of this effect by means of a combination of x-ray diffraction and x-ray photoelectron spectroscopy. Both e-beam and resistively grown gold thin films are characterized to find the subtle differences responsible for this intriguing stress behaviour. Somewhat remarkably, these studies show a tight relation between the surface structure of the gold overlayer and the SH-DNA immobilization efficiency. The average grain size variation seems to correlate well with the differential surface stress triggered by the SH-DNA immobilization previously reported. These results suggest that the relation of the probe molecules with the surface structure must be considered to understand surface stress changes.

Microcantilevers bend to the pressure of clustered redox centers

The redox-activated deflection of microcantilevers has attracted interest for nanoactuation and chemical sensing. Microcantilever sensors are devices that transduce (bio)chemical reactions into a quantifiable nanomechanical motion via surface stress changes. Despite promising applications in analytical science, poor signal-to-noise ratios and a limited understanding of the molecular origins of the surface stress changes that cause the observed deflections remain obstacles to cantilever-based sensing becoming an established (bio)detection method, such as surface plasmon resonance and electrochemistry. We use phase-separated, binary self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate and n-undecanethiolate as a model system to study the effect of the steric crowding of the redox centers on the surface stress change and cantilever deflection produced by the electrochemical oxidation of the surface-tethered ferrocene to ferrocenium. We correlate the measured surface stress change to the fraction of the clustered ferrocenyldodecanethiolate phase in the binary SAMs. The pairing of anions with the sterically crowded clustered ferroceniums induces a collective molecular reorientation which drives the cantilever deflection. The results provide fundamental insights into the response mechanism of microcantilever-based actuating and sensing technologies.