Micro- and nanomechanical sensors for environmental, chemical, and biological detection

Nanocantilevers with Adjustable Static Deflection and Significantly Tunable Spectrum Resonant Frequencies for Applications in Nanomechanical Mass Sensors

Nanocantilevers have become key components of nanomechanical sensors that exploit changes in their resonant frequencies or static deflection in response to the environment. It is necessary that they can operate at a given, but adjustable, resonant frequency and/or static deflection ranges. Here we propose a new class of nanocantilevers with a significantly tunable spectrum of the resonant frequencies and changeable static deflection utilizing the unique properties of a phase-transforming NiTi film sputtered on the usual nanotechnology cantilever materials. The reversible frequency tuning and the adjustable static deflection are obtained by intentionally changing the Young's modulus and the interlayer stress of the NiTi film during its phase transformation, while the usual cantilever elastic materials guarantee a high frequency actuation (up to tens of MHz). By incorporating the NiTi phase transformation characteristic into the classical continuum mechanics theory we present theoretical models that account for the nanocantilever frequency shift and variation in static deflection caused by a phase transformation of NiTi film. Due to the practical importance in nanomechanical sensors, we carry out a complete theoretical analysis and evaluate the impact of NiTi film on the cantilever Young's modulus, static deflection, and the resonant frequencies. Moreover, the importance of proposed NiTi nanocantilever is illustrated on the nanomechanical based mass sensors. Our findings will be of value in the development of advanced nanotechnology sensors with intentionally-changeable physical and mechanical properties.

Review: Electrostatically actuated nanobeam-based nanoelectromechanical switches - materials solutions and operational conditions

This review summarizes relevant research in the field of electrostatically actuated nanobeam-based nanoelectromechanical (NEM) switches. The main switch architectures and structural elements are briefly described and compared. Investigation methods that allow for exploring coupled electromechanical interactions as well as studies of mechanically or electrically induced effects are covered. An examination of the complex nanocontact behaviour during various stages of the switching cycle is provided. The choice of the switching element and the electrode is addressed from the materials perspective, detailing the benefits and drawbacks for each. An overview of experimentally demonstrated NEM switching devices is provided, and together with their operational parameters, the reliability issues and impact of the operating environment are discussed. Finally, the most common NEM switch failure modes and the physical mechanisms behind them are reviewed and solutions proposed.

Micro-optics for microfluidic analytical applications

This critical review summarizes the developments in the integration of micro-optical elements with microfluidic platforms for facilitating detection and automation of bio-analytical applications.

Actuation of higher harmonics in large arrays of micromechanical cantilevers for expanded resonant peak separation

A large array of elastically coupled micro cantilevers of variable length is studied experimentally and numerically. Full-scale finite element modal analysis is implemented to determine the spectral behavior of the array and to extract a global coupling matrix. A compact reduced order model is used for numerical investigation of the array's dynamic response. Our model results show that at a given excitation frequency within a propagation band, only a finite number of beams respond. Spectral characteristics of individual cantilevers, inertially excited by an external piezoelectric actuator, were measured in vacuum using laser interferometry. The theoretical and experimental results collectively show that the resonant peaks corresponding to individual beams are clearly separated when operating in vacuum at the 3^rd harmonic. Distinct resonant peak separation, coupled with the spatially-confined modal response, make higher harmonic operation of tailored, variable-length cantilever arrays well suited for a variety of resonant based sensing applications.

Probing the chaotic boundary of a membrane resonator with nanowire arrays

Mechanically induced nonlinearities in nano-electromechanical systems (NEMSs) are typically avoided in design due to their unpredictable nature; however, by incorporating these normally unwanted nonlinear and chaotic phenomena, the performance of NEMS devices displays substantially different characteristics opening a broad new range of potential applications for their use. In this work, experiments have been conducted for probing the chaotic boundary of a circular membrane mechanical resonator with and without a silicone nanowire array (Si NWA). The NWA resonator can transition from linear to nonlinear quasi-periodic behaviour, and further transition into a chaotic state at resonance. Moreover, the NWA resonator demonstrated a high level of complex nonlinear behaviours, as the device expands the power spectral response from a single frequency at a linear regime to a wideband continuous frequency spectrum when chaotic behaviour was initiated; the threshold power of this transition decreased with a smaller NWA diameter. It was also observed that the NWA resonator had higher damping compared to the resonator without a NWA; however, as the vibration velocity of the NWA resonator increased, complex air damping and thin squeeze film damping lowered the threshold for probing the chaotic boundary condition of the NWA resonator.

Nano-Integrated Suspended Polymeric Microfluidics (SPMF) Platform for Ultra-Sensitive Bio-Molecular Recognition of Bovine Growth Hormones

The development of sensitive platforms for the detection of biomolecules recognition is an extremely important problem in clinical diagnostics. In microcantilever (MC) transducers, surface-stress is induced upon bimolecular interaction which is translated into MC deflection. This paper presents a cost-effective and ultra-sensitive MC-based biosensing platform. To address these goals, the need for costly high-resolution read-out system has been eliminated by reducing the cantilever compliance through developing a polymer-based cantilever. Furthermore a microfluidic system has been integrated with the MC in order to enhance sensitivity and response time and to reduce analytes consumption. Gold nanoparticles (AuNPs) are synthesized on the surface of suspended microfluidics as the selective layer for biomolecule immobilization. The biosensing results show significant improvement in the sensitivity of the proposed platform compared with available silicon MC biosensor. A detection limit of 2 ng/ml (100pM) is obtained for the detection of bovine growth hormones. The results validated successful application of suspended polymeric microfluidics (SPMF) as the next generation of biosensing platforms which could enable femtomolar (fM) biomolecular recognition detection.

An Advanced Characterization Method for the Elastic Modulus of Nanoscale Thin-Films Using a High-Frequency Micromechanical Resonator

Nanoscale materials have properties that frequently differ from those of their bulk form due to the scale effect, and therefore a measurement technique that can take account of such material characteristics with high accuracy and sensitivity is required. In the present study, advanced nanomechanical metrology was developed for evaluation of elastic properties of thin-film materials. A 52 nm thick chromium (Cr) film was deposited on a high-speed micromechanical resonator using an e-beam evaporator, and the structure was excited to resonate using an ultrasonic platform. The resonant frequencies for the first and second flexural vibration modes were measured using laser interferometry, and they were compared to analytical estimation from the classical beam theory. Results show that the experimental data are in excellent agreement with the theory, within 1% of the relative error, and a mass sensitivity up to 10.5 Hz/fg was achieved. Thus, the scale effect that reduced the Young’s modulus of Cr by 49.8% compared to its bulk property was correctly recognized by the proposed method.

Blood Droplet-Based Cancer Diagnosis via Proteolytic Activity Measurement in Cancer Progression

Matrix metalloproteinase (MMP) is a key marker and target molecule for cancer diagnosis, as MMP is able to cleave peptide chains resulting in degradation of extracellular matrix (ECM), a necessary step for cancer development. In particular, MMP2 has recently been recognized as an important biomarker for lung cancer. Despite the important role of detecting MMP molecules in cancer diagnosis, it is a daunting task to quantitatively understand a correlation between the status of cancer development and the secretion level of MMP in a blood droplet. Here, we demonstrate a nanoscale cancer diagnosis by nanomechanical quantitation of MMP2 molecules under cancer progression with using a blood droplet of lung cancer patients. Specifically, we measured the frequency dynamics of nanomechanical biosensor functionalized with peptide chains mimicking ECM in response to MMP2 secreted from tumors in lung with different metastasis level. It is shown that the frequency shift of the biosensor, which exhibits the detection sensitivity below 1 nM, enables the quantitation of the secretion level of MMP2 molecules during the progression of cancer cells or tumor growth. More importantly, using a blood droplet of lung cancer patients, nanomechanical biosensor is shown to be capable of depicting the correlation between the secretion level of MMP2 molecules and the level of cancer metastasis, which highlights the cantilever-based MMP2 detection for diagnosis of lung cancer. Our finding will broaden the understanding of cancer development activated by MMP and allow for a fast and point-of-care cancer diagnostics.

Characterization of forced localization of disordered weakly coupled micromechanical resonators

The mode localization phenomenon of disordered weakly coupled resonators (WCRs) is being used as a novel transduction scheme to further enhance the sensitivity of micromechanical resonant sensors. In this paper, two novel characteristics of mode localization are described. First, we found that the anti-resonance loci behave as a linear function of the stiffness perturbation. The anti-resonance behavior can be regarded as a new manifestation of mode localization in the frequency domain, and mode localization occurs at a deeper level as the anti-resonance approaches closer to the resonance. The anti-resonance loci can be used to identify the symmetry of the WCRs and the locations of the perturbation. Second, by comparing the forced localization responses of the WCRs under both the single-resonator-driven (SRD) scheme and the double-resonator-driven (DRD) scheme, we demonstrated that the DRD scheme extends the linear measurement scale while sacrificing a certain amount of sensitivity. We also demonstrated experimentally that the amplitude ratio-based sensitivity under the DRD scheme is approximately an order of magnitude lower than that under the SRD scheme, that is, the amplitude ratio-based sensitivity is -70.44% (N m-1)-1 under the DRD scheme, while it is -785.6% (N m-1)-1 under the SRD scheme. These characteristics of mode localization are valuable for the design and control of WCR-based sensors.

Acoustic biosensors

Resonant and acoustic wave devices have been researched for several decades for application in the gravimetric sensing of a variety of biological and chemical analytes. These devices operate by coupling the measurand (e.g. analyte adsorption) as a modulation in the physical properties of the acoustic wave (e.g. resonant frequency, acoustic velocity, dissipation) that can then be correlated with the amount of adsorbed analyte. These devices can also be miniaturized with advantages in terms of cost, size and scalability, as well as potential additional features including integration with microfluidics and electronics, scaled sensitivities associated with smaller dimensions and higher operational frequencies, the ability to multiplex detection across arrays of hundreds of devices embedded in a single chip, increased throughput and the ability to interrogate a wider range of modes including within the same device. Additionally, device fabrication is often compatible with semiconductor volume batch manufacturing techniques enabling cost scalability and a high degree of precision and reproducibility in the manufacturing process. Integration with microfluidics handling also enables suitable sample pre-processing/separation/purification/amplification steps that could improve selectivity and the overall signal-to-noise ratio. Three device types are reviewed here: (i) bulk acoustic wave sensors, (ii) surface acoustic wave sensors, and (iii) micro/nano-electromechanical system (MEMS/NEMS) sensors.

Bioengineering Microgels and Hydrogel Microparticles for Sensing Biomolecular Targets

Hydrogels, and in particular microgels, are playing an increasingly important role in a diverse range of applications due to their hydrophilic, biocompatible, and highly flexible chemical characteristics. On this basis, solution-like environment, non-fouling nature, easy probe accessibility and target diffusion, effective inclusion of reporting moieties can be achieved, making them ideal substrates for bio-sensing applications. In fact, hydrogels are already successfully used in immunoassays as well as sensitive nucleic acid assays, also enabling hydrogel-based suspension arrays. In this review, we discuss key parameters of hydrogels in the form of micron-sized particles to be used in sensing applications, paying attention to the protein and oligonucleotides (i.e., miRNAs) targets as most representative kind of biomarkers.

Microfluidic approaches for isolation, detection, and characterization of extracellular vesicles: Current status and future directions

Extracellular vesicles (EVs) are cell-derived vesicles present in body fluids that play an essential role in various cellular processes, such as intercellular communication, inflammation, cellular homeostasis, survival, transport, and regeneration. Their isolation and analysis from body fluids have a great clinical potential to provide information on a variety of disease states such as cancer, cardiovascular complications and inflammatory disorders. Despite increasing scientific and clinical interest in this field, there are still no standardized procedures available for the purification, detection, and characterization of EVs. Advances in microfluidics allow for chemical sampling with increasingly high spatial resolution and under precise manipulation down to single molecule level. In this review, our objective is to give a brief overview on the working principle and examples of the isolation and detection methods with the potential to be used for extracellular vesicles. This review will also highlight the integrated on-chip systems for isolation and characterization of EVs.

Experimental evidence of Fano resonances in nanomechanical resonators

Fano resonance refers to an interference between localized and continuum states that was firstly reported for atomic physics and solid-state quantum devices. In recent years, Fano interference gained more and more attention for its importance in metamaterials, nanoscale photonic devices, plasmonic nanoclusters and surface-enhanced Raman scattering (SERS). Despite such interest in nano-optics, no experimental evidence of Fano interference was reported up to now for purely nanomechanical resonators, even if classical mechanical analogies were referred from a theoretical point of view. Here we demonstrate for the first time that harmonic nanomechanical resonators with relatively high quality factors, such as cantilevers vibrating in vacuum, can show characteristic Fano asymmetric curves when coupled in arrays. The reported findings open new perspectives in fundamental aspects of classical nanomechanical resonators and pave the way to a new generation of chemical and biological nanoresonator sensors with higher parallelization capability.

Sensing of single electrons using micro and nano technologies: a review

During the last three decades, the remarkable dynamic features of microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS), and advances in solid-state electronics hold much potential for the fabrication of extremely sensitive charge sensors. These sensors have a broad range of applications, such as those involving the measurement of ionization radiation, detection of bio-analyte and aerosol particles, mass spectrometry, scanning tunneling microscopy, and quantum computation. Designing charge sensors (also known as charge electrometers) for electrometry is deemed significant because of the sensitivity and resolution issues in the range of micro- and nano-scales. This article reviews the development of state-of-the-art micro- and nano-charge sensors, and discusses their technological challenges for practical implementation.

A novel classification of prostate specific antigen (PSA) biosensors based on transducing elements

During the last few decades, there has been a tremendous rise in the number of research studies dedicated towards the development of diagnostic tools based on bio-sensing technology for the early detection of various diseases like cardiovascular diseases (CVD), many types of cancer, diabetes mellitus (DM) and many infectious diseases. Many breakthroughs have been developed in the areas of improving specificity, selectivity and repeatability of the biosensor devices. Innovations in the interdisciplinary areas like biotechnology, genetics, organic electronics and nanotechnology also had a great positive impact on the growth of bio-sensing technology. As a product of these improvements, fast and consistent sensing policies have been productively created for precise and ultrasensitive biomarker-based disease diagnostics. Prostate-specific antigen (PSA) is widely considered as an important biomarker used for diagnosing prostate cancer. There have been many publications based on various biosensors used for PSA detection, but a limited review was available for the classification of these biosensors used for the detection of PSA. This review highlights the various biosensors used for PSA detection and proposes a novel classification for PSA biosensors based on the transducer type used. We also highlight the advantages, disadvantages and limitations of each technique used for PSA biosensing which will make this article a complete reference tool for the future researches in PSA biosensing.

Capacitive Biosensors and Molecularly Imprinted Electrodes

Capacitive biosensors belong to the group of affinity biosensors that operate by registering direct binding between the sensor surface and the target molecule. This type of biosensors measures the changes in dielectric properties and/or thickness of the dielectric layer at the electrolyte/electrode interface. Capacitive biosensors have so far been successfully used for detection of proteins, nucleotides, heavy metals, saccharides, small organic molecules and microbial cells. In recent years, the microcontact imprinting method has been used to create very sensitive and selective biorecognition cavities on surfaces of capacitive electrodes. This chapter summarizes the principle and different applications of capacitive biosensors with an emphasis on microcontact imprinting method with its recent capacitive biosensor applications.

Femtogram scale high frequency nano-optomechanical resonators in water

A femtogram scale nanobeam optomechanical crystal (OMC) resonator operating in water is designed and demonstrated. After immersing the device in water, the mechanical Q-factor reduces to 6.6 from 2285 in air. The thermomechanical motion of the highly damped mechanical resonance in water can be resolved using the sensitive cavity optomechanical readout. The mechanical frequency is shifted to 5.251 GHz from 5.3 GHz in air due to the added motional inertia. From the thermomechanical noise spectrum of the mechanical resonance, a noise floor of 9.33am/Hz is achieved in water. Through 2D finite element method (FEM) simulations, the acoustic dissipation dominates the low mechanical Q-factor of the device during the interaction between the mechanical resonance and surrounding water. The mass sensitivity of the present device is estimated to be 1.33ag/Hz in water.

Nanomechanical sensor applied to blood culture pellets: a fast approach to determine the antibiotic susceptibility against agents of bloodstream infections

OBJECTIVES

The management of bloodstream infection, a life-threatening disease, largely relies on early detection of infecting microorganisms and accurate determination of their antibiotic susceptibility to reduce both mortality and morbidity. Recently we developed a new technique based on atomic force microscopy capable of detecting movements of biologic samples at the nanoscale. Such sensor is able to monitor the response of bacteria to antibiotic's pressure, allowing a fast and versatile susceptibility test. Furthermore, rapid preparation of a bacterial pellet from a positive blood culture can improve downstream characterization of the recovered pathogen as a result of the increased bacterial concentration obtained.

METHODS

Using artificially inoculated blood cultures, we combined these two innovative procedures and validated them in double-blind experiments to determine the susceptibility and resistance of Escherichia coli strains (ATCC 25933 as susceptible and a characterized clinical isolate as resistant strain) towards a selection of antibiotics commonly used in clinical settings.

RESULTS

On the basis of the variance of the sensor movements, we were able to positively discriminate the resistant from the susceptible E. coli strains in 16 of 17 blindly investigated cases. Furthermore, we defined a variance change threshold of 60% that discriminates susceptible from resistant strains.

CONCLUSIONS

By combining the nanomotion sensor with the rapid preparation method of blood culture pellets, we obtained an innovative, rapid and relatively accurate method for antibiotic susceptibility test directly from positive blood culture bottles, without the need for bacterial subculture.

Micro-electromechanical film bulk acoustic sensor for plasma and whole blood coagulation monitoring

Monitoring blood coagulation is an important issue in the surgeries and the treatment of cardiovascular diseases. In this work, we reported a novel strategy for the blood coagulation monitoring based on a micro-electromechanical film bulk acoustic resonator. The resonator was excited by a lateral electric field and operated under the shear mode with a frequency of 1.9GHz. According to the apparent step-ladder curves of the frequency response to the change of blood viscoelasticity, the coagulation time (prothrombin time) and the coagulation kinetics were measured with the sample consumption of only 1μl. The procoagulant activity of thromboplastin and the anticoagulant effect of heparin on the blood coagulation process were illustrated exemplarily. The measured prothrombin times showed a good linear correlation with R²=0.99969 and a consistency with the coefficient of variation less than 5% compared with the commercial coagulometer. The proposed film bulk acoustic sensor, which has the advantages of small size, light weight, low cost, simple operation and little sample consumption, is a promising device for miniaturized, online and automated analytical system for routine diagnostics of hemostatic status and personal health monitoring.

Displacement Sensing Based on Resonant Frequency Monitoring of Electrostatically Actuated Curved Micro Beams

The ability to control nonlinear interactions of suspended mechanical structures offers a unique opportunity to engineer rich dynamical behavior that extends the dynamic range and ultimate device sensitivity. We demonstrate a displacement sensing technique based on resonant frequency monitoring of curved, doubly clamped, bistable micromechanical beams interacting with a movable electrode. In this configuration, the electrode displacement influences the nonlinear electrostatic interactions, effective stiffness and frequency of the curved beam. Increased sensitivity is made possible by dynamically operating the beam near the snap-through bistability onset. Various in-plane device architectures were fabricated from single crystal silicon and measured under ambient conditions using laser Doppler vibrometry. In agreement with the reduced order Galerkin-based model predictions, our experimental results show a significant resonant frequency reduction near critical snap-through, followed by a frequency increase within the post-buckling configuration. Interactions with a stationary electrode yield a voltage sensitivity up to ≈ 560 Hz/V and results with a movable electrode allow motion sensitivity up to ≈ 1.5 Hz/nm. Our theoretical and experimental results collectively reveal the potential of displacement sensing using nonlinear interactions of geometrically curved beams near instabilities, with possible applications ranging from highly sensitive resonant inertial detectors to complex optomechanical platforms providing an interface between the classical and quantum domains.

Serial weighting of micro-objects with resonant microchanneled cantilevers

Atomic force microscopy (AFM) cantilevers have proven to be very effective mass sensors. The attachment of a small mass to a vibrating cantilever produces a resonance frequency shift that can be monitored, providing the ability to measure mass changes down to a few molecules resolution. Nevertheless, the lack of a practical method to handle the catch and release process required for dynamic weighting of microobjects strongly hindered the application of the technology beyond proof of concept measurements. Here, a method is proposed in which FluidFM hollow cantilevers are exploited to overcome the standard limitations of AFM-based mass sensors, providing high throughput single object weighting with picogram accuracy. The extension of the dynamic models of AFM cantilevers to hollow cantilevers was discussed and the effectiveness of mass weighting in air was validated on test samples.

Fungal disease detection in plants: Traditional assays, novel diagnostic techniques and biosensors

Fungal diseases in commercially important plants results in a significant reduction in both quality and yield, often leading to the loss of an entire plant. In order to minimize the losses, it is essential to detect and identify the pathogens at an early stage. Early detection and accurate identification of pathogens can control the spread of infection. The present article provides a comprehensive overview of conventional methods, current trends and advances in fungal pathogen detection with an emphasis on biosensors. Traditional techniques are the "gold standard" in fungal detection which relies on symptoms, culture-based, morphological observation and biochemical identifications. In recent times, with the advancement of biotechnology, molecular and immunological approaches have revolutionized fungal disease detection. But the drawback lies in the fact that these methods require specific and expensive equipments. Thus, there is an urgent need for rapid, reliable, sensitive, cost effective and easy to use diagnostic methods for fungal pathogen detection. Biosensors would become a promising and attractive alternative, but they still have to be subjected to some modifications, improvements and proper validation for on-field use.

Self-Powered Triboelectric Micro Liquid/Gas Flow Sensor for Microfluidics

Liquid and gas flow sensors are important components of the micro total analysis systems (μTAS) for modern analytical sciences. In this paper, we proposed a self-powered triboelectric microfluidic sensor (TMS) by utilizing the signals produced from the droplet/bubble via the capillary and the triboelectrification effects on the liquid/solid interface for real-time liquid and gas flow detection. By alternating capillary with different diameters, the sensor's detecting range and sensitivity can be adjusted. Both the relationship between the droplet/bubble and capillary size, and the output signal of the sensor are systematically studied. By demonstrating the monitoring of the transfusion process for a patient and the gas flow produced from an injector, it shows that TMS has a great potential in building a self-powered micro total analysis system.

Carbon Nanomembranes

Carbon nanomembranes (CNMs) are synthetic 2D carbon sheets with tailored physical or chemical properties. These depend on the structure, molecular composition, and surroundings on either side. Due to their molecular thickness, they can be regarded as "interfaces without bulk" separating regions of different gaseous, liquid, or solid components and controlling the materials exchange between them. Here, a universal scheme for the fabrication of 1 nm-thick, mechanically stable, functional CNMs is presented. CNMs can be further modified, for example perforated by ion bombardment or chemically functionalized by the binding of other molecules onto the surfaces. The underlying physical and chemical mechanisms are described, and examples are presented for the engineering of complex surface architectures, e.g., nanopatterns of proteins, fluorescent dyes, or polymer brushes. A simple transfer procedure allows CNMs to be placed on various support structures, which makes them available for diverse applications: supports for electron and X-ray microscopy, nanolithography, nanosieves, Janus nanomembranes, polymer carpets, complex layered structures, functionalization of graphene, novel nanoelectronic and nanomechanical devices. To close, the potential of CNMs in filtration and sensorics is discussed. Based on tests for the separation of gas molecules, it is argued that ballistic membranes may play a prominent role in future efforts of materials separation.

Cavity optomechanical spring sensing of single molecules

Label-free bio-sensing is a critical functionality underlying a variety of health- and security-related applications. Micro-/nano-photonic devices are well suited for this purpose and have emerged as promising platforms in recent years. Here we propose and demonstrate an approach that utilizes the optical spring effect in a high-Q coherent optomechanical oscillator to dramatically enhance the sensing resolution by orders of magnitude compared with conventional approaches, allowing us to detect single bovine serum albumin proteins with a molecular weight of 66 kDa at a signal-to-noise ratio of 16.8. The unique optical spring sensing approach opens up a distinctive avenue that not only enables biomolecule sensing and recognition at individual level, but is also of great promise for broad physical sensing applications that rely on sensitive detection of optical cavity resonance shift to probe external physical parameters.

Influence of Sample Volume and Solvent Evaporation on Absorbance Spectroscopy in a Microfluidic "Pillar-Cuvette"

Spectroscopic analysis of solutions containing samples at high concentrations or molar absorptivity can present practical challenges. In absorbance spectroscopy, short optical path lengths or multiple dilution is required to bring the measured absorbance into the range of the Beer's Law calibration. We have previously reported an open "pillar-cuvette" with a micropillar array that is spontaneously filled with a precise (nL or μL) volume to create the well-defined optical path of, for example, 10 to 20 μm. Evaporation should not be ignored for open cuvettes and, herein, the volume of loaded sample and the rate of evaporation from the cuvette are studied. It was found that the volume of loaded sample (between 1 and 10 μL) had no effect on the Beer's Law calibration for methyl orange solutions (molar absorptivity of (2.42 ± 0.02)× 10(4) L mol(-1) cm(-1)) for cuvettes with a 14.2 ± 0.2 μm path length. Evaporation rates of water from methyl orange solutions were between 2 and 5 nL s(-1) (30 - 40% relative humidity; 23°C), depending on the sample concentration and ambient conditions. Evaporation could be reduced by placing a cover slip several millimeters above the cuvette. Importantly, the results show that a "drop-and-measure" method (measurement within ∼3 s of cuvette loading) eliminates the need for extrapolation of the absorbance-time data for accurate analysis of samples.

A deep etching mechanism for trench-bridging silicon nanowires

Introducing a single silicon nanowire with a known orientation and dimensions to a specific layout location constitutes a major challenge. The challenge becomes even more formidable, if one chooses to realize the task in a monolithic fashion with an extreme topography, a characteristic of microsystems. The need for such a monolithic integration is fueled by the recent surge in the use of silicon nanowires as functional building blocks in various electromechanical and optoelectronic applications. This challenge is addressed in this work by introducing a top-down, silicon-on-insulator technology. The technology provides a pathway for obtaining well-controlled silicon nanowires along with the surrounding microscale features up to a three-order-of-magnitude scale difference. A two-step etching process is developed, where the first shallow etch defines a nanoscale protrusion on the wafer surface. After applying a conformal protection on the protrusion, a deep etch step is carried out forming the surrounding microscale features. A minimum nanowire cross-section of 35 nm by 168 nm is demonstrated in the presence of an etch depth of 10 μm. Nanowire cross-sectional features are characterized via transmission electron microscopy and linked to specific process steps. The technology allows control on all dimensional aspects along with the exact location and orientation of the silicon nanowire. The adoption of the technology in the fabrication of micro and nanosystems can potentially lead to a significant reduction in process complexity by facilitating direct access to the nanowire during surface processes such as contact formation and doping.

Nanotools and molecular techniques to rapidly identify and fight bacterial infections

Reducing the emergence and spread of antibiotic-resistant bacteria is one of the major healthcare issues of our century. In addition to the increased mortality, infections caused by multi-resistant bacteria drastically enhance the healthcare costs, mainly because of the longer duration of illness and treatment. While in the last 20years, bacterial identification has been revolutionized by the introduction of new molecular techniques, the current phenotypic techniques to determine the susceptibilities of common Gram-positive and Gram-negative bacteria require at least two days from collection of clinical samples. Therefore, there is an urgent need for the development of new technologies to determine rapidly drug susceptibility in bacteria and to achieve faster diagnoses. These techniques would also lead to a better understanding of the mechanisms that lead to the insurgence of the resistance, greatly helping the quest for new antibacterial systems and drugs. In this review, we describe some of the tools most currently used in clinical and microbiological research to study bacteria and to address the challenge of infections. We discuss the most interesting advancements in the molecular susceptibility testing systems, with a particular focus on the many applications of the MALDI-TOF MS system. In the field of the phenotypic characterization protocols, we detail some of the most promising semi-automated commercial systems and we focus on some emerging developments in the field of nanomechanical sensors, which constitute a step towards the development of rapid and affordable point-of-care testing devices and techniques. While there is still no innovative technique that is capable of completely substituting for the conventional protocols and clinical practices, many exciting new experimental setups and tools could constitute the basis of the standard testing package of future microbiological tests.

Comparison of the specificity and affinity of surface immobilised Affimer binders using the quartz crystal microbalance

This study investigates the performance of surface bound Affimer proteins, comparing the affinity and specificity of different binders for closely related immunoglobulin molecules using the quartz crystal microbalance with dissipation monitoring (QCM-D).

Metal ion binding properties of a bimodal triazolyl-functionalized calix[4]arene on a multi-array microcantilever system. Synthesis, fluorescence and DFT computation studies

A bimodal calix[4]arene functionalized with triazolyl-linked anthracenyl and 3-propylthioacetate groups is described.

Molecular dynamics study of pressure-driven water transport through graphene bilayers

Water molecules form layered structures inside graphene bilayers and ultra-high pressure-driven flow rates can be observed.

Self-Powered Triboelectric Nanosensor for Microfluidics and Cavity-Confined Solution Chemistry

Micro total analysis system (μTAS) is one of the important tools for modern analytical sciences. In this paper, we not only propose the concept of integrating the self-powered triboelectric microfluidic nanosensor (TMN) with μTAS, but also demonstrate that the developed system can be used as an in situ tool to quantify the flowing liquid for microfluidics and solution chemistry. The TMN automatically generates electric outputs when the fluid passing through it and the outputs are affected by the solution temperature, polarity, ionic concentration, and fluid flow velocity. The self-powered TMN can detect the flowing water velocity, position, reaction temperature, ethanol, and salt concentrations. We also integrate the TMNs in a μTAS platform to directly characterize the synthesis of Au nanoparticles by a chemical reduction method.

Real-Time Label-Free Direct Electronic Monitoring of Topoisomerase Enzyme Binding Kinetics on Graphene

Monolayer graphene field-effect sensors operating in liquid have been widely deployed for detecting a range of analyte species often under equilibrium conditions. Here we report on the real-time detection of the binding kinetics of the essential human enzyme, topoisomerase I interacting with substrate molecules (DNA probes) that are immobilized electrochemically on to monolayer graphene strips. By monitoring the field-effect characteristics of the graphene biosensor in real-time during the enzyme-substrate interactions, we are able to decipher the surface binding constant for the cleavage reaction step of topoisomerase I activity in a label-free manner. Moreover, an appropriate design of the capture probes allows us to distinctly follow the cleavage step of topoisomerase I functioning in real-time down to picomolar concentrations. The presented results are promising for future rapid screening of drugs that are being evaluated for regulating enzyme activity.

Permanent reduction of dissipation in nanomechanical Si resonators by chemical surface protection

We report on mechanical dissipation measurements carried out on thin (∼100 nm), single-crystal silicon cantilevers with varying chemical surface termination. We find that the 1-2 nm-thick native oxide layer of silicon contributes about 85% to the friction of the mechanical resonance. We show that the mechanical friction is proportional to the thickness of the oxide layer and that it crucially depends on oxide formation conditions. We further demonstrate that chemical surface protection by nitridation, liquid-phase hydrosilylation, or gas-phase hydrosilylation can inhibit rapid oxide formation in air and results in a permanent improvement of the mechanical quality factor between three- and five-fold. This improvement extends to cryogenic temperatures. Presented recipes can be directly integrated with standard cleanroom processes and may be especially beneficial for ultrasensitive nanomechanical force- and mass sensors, including silicon cantilevers, membranes, or nanowires.

Suspended photonic waveguide devices

This article describes recent research at the U.S. Naval Research Laboratory that focuses on the use of micro- and nanomachining techniques for photonic waveguide devices. By selectively etching a sacrificial layer that the waveguide core is supported by, in whole or in part, the waveguide obtains enhanced properties and functionality, such as mechanical flexibility, index contrast, birefringence, and evanescent field depth. We describe how these properties enable unique waveguide applications in areas such as cavity optomechanics, displacement sensing, electro-optics, and nonlinear optics.

Tunable micro- and nanomechanical resonators

Advances in micro- and nanofabrication technologies have enabled the development of novel micro- and nanomechanical resonators which have attracted significant attention due to their fascinating physical properties and growing potential applications. In this review, we have presented a brief overview of the resonance behavior and frequency tuning principles by varying either the mass or the stiffness of resonators. The progress in micro- and nanomechanical resonators using the tuning electrode, tuning fork, and suspended channel structures and made of graphene have been reviewed. We have also highlighted some major influencing factors such as large-amplitude effect, surface effect and fluid effect on the performances of resonators. More specifically, we have addressed the effects of axial stress/strain, residual surface stress and adsorption-induced surface stress on the sensing and detection applications and discussed the current challenges. We have significantly focused on the active and passive frequency tuning methods and techniques for micro- and nanomechanical resonator applications. On one hand, we have comprehensively evaluated the advantages and disadvantages of each strategy, including active methods such as electrothermal, electrostatic, piezoelectrical, dielectric, magnetomotive, photothermal, mode-coupling as well as tension-based tuning mechanisms, and passive techniques such as post-fabrication and post-packaging tuning processes. On the other hand, the tuning capability and challenges to integrate reliable and customizable frequency tuning methods have been addressed. We have additionally concluded with a discussion of important future directions for further tunable micro- and nanomechanical resonators.

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.

High-frequency nano-optomechanical disk resonators in liquids

Nano- and micromechanical resonators are the subject of research that aims to develop ultrasensitive mass sensors for spectrometry, chemical analysis and biomedical diagnosis. Unfortunately, their merits generally diminish in liquids because of an increased dissipation. The development of faster and lighter miniaturized devices would enable improved performances, provided the dissipation was controlled and novel techniques were available to drive and readout their minute displacement. Here we report a nano-optomechanical approach to this problem using miniature semiconductor disks. These devices combine a mechanical motion at high frequencies (gigahertz and above) with an ultralow mass (picograms) and a moderate dissipation in liquids. We show that high-sensitivity optical measurements allow their Brownian vibrations to be resolved directly, even in the most-dissipative liquids. We investigate their interaction with liquids of arbitrary properties, and analyse measurements in light of new models. Nano-optomechanical disks emerge as probes of rheological information of unprecedented sensitivity and speed, which opens up applications in sensing and fundamental science.

Diazonium Chemistry for the Bio-Functionalization of Glassy Nanostring Resonator Arrays

Resonant glassy nanostrings have been employed for the detection of biomolecules. These devices offer high sensitivity and amenability to large array integration and multiplexed assays. Such a concept has however been impaired by the lack of stable and biocompatible linker chemistries. Diazonium salt reduction-induced aryl grafting is an aqueous-based process providing strong chemical adhesion. In this work, diazonium-based linker chemistry was performed for the first time on glassy nanostrings, which enabled the bio-functionalization of such devices. Large arrays of nanostrings with ultra-narrow widths down to 10 nm were fabricated employing electron beam lithography. Diazonium modification was first developed on SiCN surfaces and validated by X-ray photoelectron spectroscopy. Similarly modified nanostrings were then covalently functionalized with anti-rabbit IgG as a molecular probe. Specific enumeration of rabbit IgG was successfully performed through observation of downshifts of resonant frequencies. The specificity of this enumeration was confirmed through proper negative control experiments. Helium ion microscopy further verified the successful functionalization of nanostrings.

Functionalized Polymer Microgel Particles Enable Customizable Production of Label-Free Sensor Arrays

Probe molecule immobilization onto surfaces is a critical step in the production of many analytical devices, including labeled and label-free microarrays. New methods to increase the density and uniformity of probe deposition have the potential to significantly enhance the ultimate limits of detection and reproducibility. Hydrogel-based materials have been employed in the past to provide a 3D protein-friendly surface for deposition of antibodies and nucleic acids. However, these methods are susceptible to variation during polymerization of the hydrogel scaffold and provide limited opportunities for tuning deposition parameters on an antibody-by-antibody basis. In this work, a versatile hydrogel nanoparticle deposition method was developed for the production of label-free microarrays and tested in the context of antibody-antigen binding. Poly(N-isopropylacrylamide) nanoparticles (PNIPAM) were conjugated to antibodies using an avidin/biotin system and deposited onto surfaces using a noncontact printing system. After drying, these gel spots formed uniform and thin layers <10 nm in height. The conjugates were characterized with dynamic light scattering, scanning electron microscopy, and atomic force microscopy. We tested this format in the context of tumor necrosis factor-alpha (TNF-α) detection via arrayed imaging reflectometry (AIR), a label-free protein microarray method. This method of probe molecule deposition should be generally useful in the production of microarrays for label-free detection.

Single Nanoparticle Detection Using Far-field Emission of Photonic Molecule around the Exceptional Point

Highly sensitive, label-free detection methods have important applications in fundamental research and healthcare diagnostics. To date, the detection of single nanoparticles has remained largely dependent on extremely precise spectral measurement, which relies on high-cost equipment. Here, we demonstrate a simple but very nontrivial mechanism for the label-free sizing of nanoparticles using the far-field emission of a photonic molecule (PM) around an exceptional point (EP). By attaching a nanoparticle to a PM around an EP, the main resonant behaviors are strongly disturbed. In addition to typical mode splitting, we find that the far-field pattern of the PM is significantly changed. Taking a heteronuclear diatomic PM as an example, we demonstrate that a single nanoparticle, whose radius is as small as 1 nm to 7 nm, can be simply monitored through the variation of the far-field pattern. Compared with conventional methods, our approach is much easier and does not rely on high-cost equipment. In addition, this research will illuminate new advances in single nanoparticle detection.

Plasmomechanical Resonators Based on Dimer Nanoantennas

Nanomechanical resonators are highly suitable as sensors of minute forces, displacements, or masses. We realize a single plasmonic dimer antenna of subwavelength size, integrated with silicon nitride nanobeams. The sensitive dependence of the antenna response on the beam displacement creates a plasmomechanical system of deeply subwavelength size in all dimensions. We use it to demonstrate transduction of thermal vibrations to scattered light fields and discuss the noise properties and achievable coupling strengths in these systems.