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

Compressible Viscoelastic Liquid Effects Generated by the Breathing Modes of Isolated Metal Nanowires

Transient absorption microscopy is used to examine the breathing modes of single gold nanowires in highly viscous liquids. By performing measurements on the same wire in air and liquid, the damping contribution from the liquid can be separated from the intrinsic damping of the nanowire. The results show that viscous liquids strongly reduce the vibrational lifetimes but not to the extent predicted by standard models for nanomaterial-liquid interactions. To explain these results a general theory for compressible viscoelastic fluid-structure interactions is developed. The theory results are in good agreement with experiment, which confirms that compressible non-Newtonian flow phenomena are important for vibrating nanostructures. This is the first theoretical study and experimental measurement of the compressible viscoelastic properties of simple liquids.

Pillar cuvettes: capillary-filled, microliter quartz cuvettes with microscale path lengths for optical spectroscopy

The goal of most analytical techniques is to reduce the lower limit of detection; however, it is sometimes necessary to do the opposite. High sample concentrations or samples with high molar absorptivity (e.g., dyes and metal complexes) often require multiple dilution steps or laborious sample preparation prior to spectroscopic analysis. Here, we demonstrate dilution-free, one-step UV-vis spectroscopic analysis of high concentrations of platinum(IV) hexachloride in a micropillar array, that is, "pillar cuvette". The cuvette is spontaneously filled by wicking of the liquid sample into the micropillar array. The pillar height (thus, the film thickness) defines the optical path length, which was reduced to between 10 and 20 μm in this study (3 orders of magnitude smaller than in a typical cuvette). Only one small droplet (∼2 μL) of sample is required, and the dispensed volume need not be precise or even known to the analyst for accurate spectroscopy measurements. For opaque pillars, we show that absorbance is linearly related to platinum concentration (the Beer-Lambert Law). For fully transparent or semitransparent pillars, the measured absorbance was successfully corrected for the fractional surface coverage of the pillars and the transmittance of the pillars and reference. Thus, both opaque and transparent pillars can be applied to absorbance spectroscopy of high absorptivity, microliter samples. It is also shown here that the pillar array has a useful secondary function as an integrated (in-cuvette) filter for particulates. For pillar cuvette measurements of platinum solutions spiked with 6 μm diameter polystyrene spheres, filtered and unfiltered samples gave identical spectra.

Fabrication of buckling free ultrathin silicon membranes by direct bonding with thermal difference

An innovative method to fabricate large area (up to several squared millimeters) ultrathin (100 nm) monocrystalline silicon (Si) membranes is described. This process is based on the direct bonding of a silicon-on-insulator wafer with a preperforated silicon wafer. The stress generated by the thermal difference applied during the bonding process is exploited to produce buckling free silicon nanomembranes of large areas. The thermal differences required to achieve these membranes (≥1 mm(2)) are estimated by analytical calculations. An experimental study of the stress achievable by direct bonding through two specific surface preparations (hydrophobic or hydrophilic) is reported. Buckling free silicon nanomembranes secured on a 2 × 2 cm(2) frame with lateral dimensions up to 5 × 5 mm(2) are successfully fabricated using the optimized direct bonding process. The stress estimated by theoretical analysis is confirmed by Raman measurements, while the flatness of the nanomembranes is demonstrated by optical interferometry. The successful fabrications of high resolution (50 nm half pitch) tungsten gratings on the silicon nanomembranes and of focused ion beam milling nanostructures show the promising potential of the Si membranes for X-ray optics and for the emerging nanosensor market.

Design of surface modifications for nanoscale sensor applications

Nanoscale biosensors provide the possibility to miniaturize optic, acoustic and electric sensors to the dimensions of biomolecules. This enables approaching single-molecule detection and new sensing modalities that probe molecular conformation. Nanoscale sensors are predominantly surface-based and label-free to exploit inherent advantages of physical phenomena allowing high sensitivity without distortive labeling. There are three main criteria to be optimized in the design of surface-based and label-free biosensors: (i) the biomolecules of interest must bind with high affinity and selectively to the sensitive area; (ii) the biomolecules must be efficiently transported from the bulk solution to the sensor; and (iii) the transducer concept must be sufficiently sensitive to detect low coverage of captured biomolecules within reasonable time scales. The majority of literature on nanoscale biosensors deals with the third criterion while implicitly assuming that solutions developed for macroscale biosensors to the first two, equally important, criteria are applicable also to nanoscale sensors. We focus on providing an introduction to and perspectives on the advanced concepts for surface functionalization of biosensors with nanosized sensor elements that have been developed over the past decades (criterion (iii)). We review in detail how patterning of molecular films designed to control interactions of biomolecules with nanoscale biosensor surfaces creates new possibilities as well as new challenges.

Detecting nanoscale vibrations as signature of life

The existence of life in extreme conditions, in particular in extraterrestrial environments, is certainly one of the most intriguing scientific questions of our time. In this report, we demonstrate the use of an innovative nanoscale motion sensor in life-searching experiments in Earth-bound and interplanetary missions. This technique exploits the sensitivity of nanomechanical oscillators to transduce the small fluctuations that characterize living systems. The intensity of such movements is an indication of the viability of living specimens and conveys information related to their metabolic activity. Here, we show that the nanomotion detector can assess the viability of a vast range of biological specimens and that it could be the perfect complement to conventional chemical life-detection assays. Indeed, by combining chemical and dynamical measurements, we could achieve an unprecedented depth in the characterization of life in extreme and extraterrestrial environments.

Hybrid spin-microcantilever sensor for environmental, chemical, and biological detection

Nowadays hybrid spin-micro/nanomechanical systems are being actively explored for potential quantum sensing applications. In combination with the pump-probe technique or the spin resonance spectrum, we theoretically propose a realistic, feasible, and an exact way to measure the cantilever frequency in a hybrid spin-micromechanical cantilever system which has a strong coherent coupling of a single nitrogen vacancy center in the single-crystal diamond cantilever with the microcantilever. The probe absorption spectrum which exhibits new features such as mechanically induced three-photon resonance and ac Stark effect is obtained. Simultaneously, we further develop this hybrid spin-micromechanical system to be an ultrasensitive mass sensor, which can be operated at 300 K with a mass responsivity 0.137 Hz ag(-1), for accurate sensing of gaseous or aqueous environments, chemical vapors, and biomolecules. And the best performance on the minimum detectable mass can be [Formula: see text] in vacuum. Finally, we illustrate an in situ measurement to detect Angiopoietin-1, a marker of tumor angiogenesis, accurately with this hybrid microcantilever at room temperature.

Adjustable plasmonic optical properties of hollow gold nanospheres monolayers and LSPR-dependent surface-enhanced Raman scattering of hollow gold nanosphere/graphene oxide hybrids

Colloidal hollow gold nanospheres with adjustable localized surface plasmon resonance (LSPR) properties were synthesized and self-assembled into HGNs monolayers for investigation of LSPR-dependent surface enhanced Raman scattering (SERS) behavior.

Parallel Transduction of Nanomechanical Motion Using Plasmonic Resonators

We demonstrate parallel transduction of thermally driven mechanical motion of an array of gold-coated silicon nitride nanomechanical beams, by using near-field confinement in plasmonic metal-insulator-metal resonators supported in the gap between the gold layers. The free-space optical readout, enabled by the plasmonic resonances, allows for addressing multiple mechanical resonators in a single measurement. Light absorbed in the metal layer of the beams modifies their mechanical properties, allowing photothermal tuning of the eigenfrequencies. The appearance of photothermally driven parametric amplification indicates the possibility of plasmonic mechanical actuation.

Monitoring sessile droplet evaporation on a micromechanical device

A bulk acoustic mode micro-electro-mechanical dual resonator platform is utilised to study the evaporation of sub-microliter water droplets from the surface of the resonator. An analytical formulation for the observed frequency shift and the measure dependence of resonant frequency on the modes of evaporation which is consistent with the optically derived data. The resonators access only a thin layer of the liquid through shear contact and, hence, the response is not affected by the bulk mass of the droplet to first order. A relationship between the droplet contact area and the elapsed time was established for the evaporation process and is used to derive a value of the diffusion coefficient of water in air that is found to be in reasonable agreement with literature values. This work introduces a new tool for the electro-mechanical monitoring of droplet evaporation with relevance to applications such as biosensing in liquid samples of sub-microliter volumes.

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.

Recent developments in optical detection technologies in lab-on-a-chip devices for biosensing applications

The field of microfluidics has yet to develop practical devices that provide real clinical value. One of the main reasons for this is the difficulty in realizing low-cost, sensitive, reproducible, and portable analyte detection microfluidic systems. Previous research has addressed two main approaches for the detection technologies in lab-on-a-chip devices: (a) study of the compatibility of conventional instrumentation with microfluidic structures, and (b) integration of innovative sensors contained within the microfluidic system. Despite the recent advances in electrochemical and mechanical based sensors, their drawbacks pose important challenges to their application in disposable microfluidic devices. Instead, optical detection remains an attractive solution for lab-on-a-chip devices, because of the ubiquity of the optical methods in the laboratory. Besides, robust and cost-effective devices for use in the field can be realized by integrating proper optical detection technologies on chips. This review examines the recent developments in detection technologies applied to microfluidic biosensors, especially addressing several optical methods, including fluorescence, chemiluminescence, absorbance and surface plasmon resonance.

Graphitized silicon carbide microbeams: wafer-level, self-aligned graphene on silicon wafers

Currently proven methods that are used to obtain devices with high-quality graphene on silicon wafers involve the transfer of graphene flakes from a growth substrate, resulting in fundamental limitations for large-scale device fabrication. Moreover, the complex three-dimensional structures of interest for microelectromechanical and nanoelectromechanical systems are hardly compatible with such transfer processes. Here, we introduce a methodology for obtaining thousands of microbeams, made of graphitized silicon carbide on silicon, through a site-selective and wafer-scale approach. A Ni-Cu alloy catalyst mediates a self-aligned graphitization on prepatterned SiC microstructures at a temperature that is compatible with silicon technologies. The graphene nanocoating leads to a dramatically enhanced electrical conductivity, which elevates this approach to an ideal method for the replacement of conductive metal films in silicon carbide-based MEMS and NEMS devices.

The role of low levels of fullerene C60 nanocrystals on enhanced learning and memory of rats through persistent CaMKII activation

Engineered nanomaterials are known to exhibit diverse and sometimes unexpected biological effects. Fullerene nanoparticles have been reported to specifically bind to and elicit persistent activation of hippocampal Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), a multimeric intracellular serine/threonine kinase central to Ca(2+) signal transduction and critical for synaptic plasticity, but the functional consequence of that modulation is unknown. Here we show that low doses of fullerene C60 nanocrystals (Nano C60), delivered through intrahippocampal infusion and without any obvious cytotoxicity in hippocampal neuronal cells, enhance the long-term potentiation (LTP) of rats. Intraperitoneal injection of 320 μg/kg of Nano C60, once daily for 10 days, also enhanced spatial memory of rats in addition to an increase of LTP. In parallel, both the IH and IP administration of Nano C60 increased the autonomous activity and the level of threonine 286 (T286) autophosphorylation of CaMKII, enhanced post-synaptic AMPA/NMDA ratio, and triggered time-dependent activation of ERK and CREB. Our results reveal a striking and highly unexpected ability of Nano C60 in positively modulating learning and memory, an effect that is most likely manifested through locking CaMKII in an active conformation, and may have significant implications for the potential therapeutic applications of fullerene C60, a classic engineered nanomaterial.

Real-time monitoring of protein conformational changes using a nano-mechanical sensor

Proteins can switch between different conformations in response to stimuli, such as pH or temperature variations, or to the binding of ligands. Such plasticity and its kinetics can have a crucial functional role, and their characterization has taken center stage in protein research. As an example, Topoisomerases are particularly interesting enzymes capable of managing tangled and supercoiled double-stranded DNA, thus facilitating many physiological processes. In this work, we describe the use of a cantilever-based nanomotion sensor to characterize the dynamics of human topoisomerase II (Topo II) enzymes and their response to different kinds of ligands, such as ATP, which enhance the conformational dynamics. The sensitivity and time resolution of this sensor allow determining quantitatively the correlation between the ATP concentration and the rate of Topo II conformational changes. Furthermore, we show how to rationalize the experimental results in a comprehensive model that takes into account both the physics of the cantilever and the dynamics of the ATPase cycle of the enzyme, shedding light on the kinetics of the process. Finally, we study the effect of aclarubicin, an anticancer drug, demonstrating that it affects directly the Topo II molecule inhibiting its conformational changes. These results pave the way to a new way of studying the intrinsic dynamics of proteins and of protein complexes allowing new applications ranging from fundamental proteomics to drug discovery and development and possibly to clinical practice.

Combination of fluorescence microscopy and nanomotion detection to characterize bacteria

Antibiotic-resistant pathogens are a major health concern in everyday clinical practice. Because their detection by conventional microbial techniques requires minimally 24 h, some of us have recently introduced a nanomechanical sensor, which can reveal motion at the nanoscale. By monitoring the fluctuations of the sensor, this technique can evidence the presence of bacteria and their susceptibility to antibiotics in less than 1 h. Their amplitude correlates to the metabolism of the bacteria and is a powerful tool to characterize these microorganisms at low densities. This technique is new and calls for an effort to optimize its protocol and determine its limits. Indeed, many questions remain unanswered, such as the detection limits or the correlation between the bacterial distribution on the sensor and the detection's output. In this work, we couple fluorescence microscopy to the nanomotion investigation to determine the optimal experimental protocols and to highlight the effect of the different bacterial distributions on the sensor.

Ultraspecific and highly sensitive nucleic acid detection by integrating a DNA catalytic network with a label-free microcavity

Nucleic acid detection with label-free biosensors circumvents costly fluorophore functionalization steps associated with conventional assays by utilizing transducers of impressive ultimate detection limits. Despite this technological prowess, molecular recognition at a surface limits the biosensors' sensitivity, specificity, and reusability. It is therefore imperative to integrate novel molecular approaches with existing label-free transducers to overcome those limitations. Here, we demonstrate this concept by integrating a DNA strand displacement circuit with a micron-scale whispering gallery mode (WGM) microsphere biosensor. The integrated biosensor exhibits at least 25-fold improved nucleic acid sensitivity, and sets a new record for label-free microcavity biosensors by detecting 80 pM (32 fmol) of a 22nt oligomer; this improvement results from the catalytic behavior of the circuit. Furthermore, the integrated sensor exhibits extremely high specificity; single nucleotide variants yield 40- to 100-fold lower signal. Finally, the same physical sensor was demonstrated to alternatingly detect 2 different nucleic acid sequences through 5 cycles of detection, showcasing both its reusability and its versatility.

Building a Fab on a Chip

Semiconductor fabs are large, complex industrial sites with costs for a single facility approaching $10B. In this paper we discuss the possibility of putting the entire functionality of such a fab onto a single silicon chip. We demonstrate a path forward where, for certain applications, especially at the nanometer scale, one can consider using a single chip approach for building devices with significant potential cost savings. In our approach, we build micro versions of the macro machines one typically finds in a fab, and integrating all the components together. We argue that the technology now exists to allow one to build a Fab on a Chip.

Hybrid integrated label-free chemical and biological sensors

Label-free sensors based on electrical, mechanical and optical transduction methods have potential applications in numerous areas of society, ranging from healthcare to environmental monitoring. Initial research in the field focused on the development and optimization of various sensor platforms fabricated from a single material system, such as fiber-based optical sensors and silicon nanowire-based electrical sensors. However, more recent research efforts have explored designing sensors fabricated from multiple materials. For example, synthetic materials and/or biomaterials can also be added to the sensor to improve its response toward analytes of interest. By leveraging the properties of the different material systems, these hybrid sensing devices can have significantly improved performance over their single-material counterparts (better sensitivity, specificity, signal to noise, and/or detection limits). This review will briefly discuss some of the methods for creating these multi-material sensor platforms and the advances enabled by this design approach.

An impedimetric study of DNA hybridization on paper-supported inkjet-printed gold electrodes

In this study, two different supramolecular recognition architectures for impedimetric detection of DNA hybridization have been formed on disposable paper-supported inkjet-printed gold electrodes. The gold electrodes were fabricated using a gold nanoparticle based ink. The first recognition architecture consists of subsequent layers of biotinylated self-assembly monolayer (SAM), streptavidin and biotinylated DNA probe. The other recognition architecture is constructed by immobilization of thiol-functionalized DNA probe (HS-DNA) and subsequent backfill with 11-mercapto-1-undecanol (MUOH) SAM. The binding capacity and selectivity of the recognition architectures were examined by surface plasmon resonance (SPR) measurements. SPR results showed that the HS-DNA/MUOH system had a higher binding capacity for the complementary DNA target. Electrochemical impedance spectroscopy (EIS) measurements showed that the hybridization can be detected with impedimetric spectroscopy in picomol range for both systems. EIS signal indicated a good selectivity for both recognition architectures, whereas SPR showed very high unspecific binding for the HS-DNA/MUOH system. The factors affecting the impedance signal were interpreted in terms of the complexity of the supramolecular architecture. The more complex architecture acts as a less ideal capacitive sensor and the impedance signal is dominated by the resistive elements.

Point of Care Technologies for HIV

Effective prevention of HIV/AIDS requires early diagnosis, initiation of therapy, and regular plasma viral load monitoring of the infected individual. In addition, incidence estimation using accurate and sensitive assays is needed to facilitate HIV prevention efforts in the public health setting. Therefore, more affordable and accessible point-of-care (POC) technologies capable of providing early diagnosis, HIV viral load measurements, and CD4 counts in settings where HIV is most prevalent are needed to enable appropriate intervention strategies and ultimately stop transmission of the virus within these populations to achieve the future goal of an AIDS-free generation. This review discusses the available and emerging POC technologies for future application to these unmet public health needs.

Rapid and low-cost prototyping of 3D nanostructures with multi-layer hydrogen silsesquioxane scaffolds

A layer-by-layer (LBL) method can generate or approximate any three-dimensional (3D) structure, and has been the approach for the manufacturing of complementary metal-oxide-semiconductor (CMOS) devices. However, its high cost precludes the fabrication of anything other than CMOS-compatible devices, and general 3D nanostructures have been difficult to prototype in academia and small businesses, due to the lack of expensive facility and state-of-the-art tools. It is proposed and demonstrated that a novel process that can rapidly fabricate high-resolution three-dimensional (3D) nanostructures at low cost, without requiring specialized equipment. An individual layer is realized through electron-beam lithography patterning of hydrogen silsesquioxane (HSQ) resist, followed by planarization via spinning SU-8 resist and etch-back. A 4-layer silicon inverse woodpile photonic crystal with a period of 650 nm and a 7-layer HSQ scaffold with a period of 300 nm are demonstrated. This process provides a versatile and accessible solution to the fabrication of highly complex 3D nanostructures.

Silicon nanowires: where mechanics and optics meet at the nanoscale

Mechanical transducers based on nanowires promise revolutionary advances in biological sensing and force microscopy/spectroscopy. A crucial step is the development of simple and non-invasive techniques able to detect displacements with subpicometer sensitivity per unit bandwidth. Here, we design suspended tapered silicon nanowires supporting a range of optical resonances that confine and efficiently scatter light in the visible range. Then, we develop an optical method for efficiently coupling the evanescent field to the regular interference pattern generated by an incoming laser beam and the reflected beam from the substrate underneath the nanowire. This optomechanical coupling is here applied to measure the displacement of 50 nm wide nanowires with sensitivity on the verge of 1 fm/Hz(1/2) at room temperature with a simple laser interferometry set-up. This method opens the door to the measurement of the Brownian motion of ultrashort nanowires for the detection of single biomolecular recognition events in liquids, and single molecule spectroscopy in vacuum.

Sensor Arrays Based on Polycyclic Aromatic Hydrocarbons: Chemiresistors versus Quartz-Crystal Microbalance

Arrays of broadly cross-reactive sensors are key elements of smart, self-training sensing systems. Chemically sensitive resistors and quartz-crystal microbalance (QCM) sensors are attractive for sensing applications that involve detection and classification of volatile organic compounds (VOCs) in the gas phase. Polycyclic aromatic hydrocarbon (PAH) derivatives as sensing materials can provide good sensitivity and robust selectivity towards different polar and nonpolar VOCs, while being quite tolerant to large humidity variations. Here, we present a comparative study of chemiresistor and QCM arrays based on a set of custom-designed PAH derivatives having either purely nonpolar coronas or alternating nonpolar and strongly polar side chain termination. The arrays were exposed to various concentrations of representative polar and nonpolar VOCs under extremely varying humidity conditions (5-80% RH). The sensor arrays' classification ability of VOC polarity, chemical class and compound separation was explained in terms of the sensing characteristics of the constituent sensors and their interaction with the VOCs. The results presented here contribute to the development of novel versatile and cost-effective real-world VOC sensing platforms.

Programmable mechanical resonances in MEMS by localized joule heating of phase change materials

A programmable micromechanical resonator based on a VO2 thin film is reported. Multiple mechanical eigenfrequency states are programmed using Joule heating as local power source, gradually driving the phase transition of VO2 around its Metal-Insulator transition temperature. Phase coexistence of domains is used to tune the stiffness of the device via local control of internal stresses and mechanical properties. This study opens perspectives for developing mechanically configurable nanostructure arrays.

A CMOS wireless biomolecular sensing system-on-chip based on polysilicon nanowire technology

As developments of modern societies, an on-field and personalized diagnosis has become important for disease prevention and proper treatment. To address this need, in this work, a polysilicon nanowire (poly-Si NW) based biosensor system-on-chip (bio-SSoC) is designed and fabricated by a 0.35 μm 2-Poly-4-Metal (2P4M) complementary metal-oxide-semiconductor (CMOS) process provided by a commercialized semiconductor foundry. Because of the advantages of CMOS system-on-chip (SoC) technologies, the poly-Si NW biosensor is integrated with a chopper differential-difference amplifier (DDA) based analog-front-end (AFE), a successive approximation analog-to-digital converter (SAR ADC), and a microcontroller to have better sensing capabilities than a traditional Si NW discrete measuring system. In addition, an on-off key (OOK) wireless transceiver is also integrated to form a wireless bio-SSoC technology. This is pioneering work to harness the momentum of CMOS integrated technology into emerging bio-diagnosis technologies. This integrated technology is experimentally examined to have a label-free and low-concentration biomolecular detection for both Hepatitis B Virus DNA (10 fM) and cardiac troponin I protein (3.2 pM). Based on this work, the implemented wireless bio-SSoC has demonstrated a good biomolecular sensing characteristic and a potential for low-cost and mobile applications. As a consequence, this developed technology can be a promising candidate for on-field and personalized applications in biomedical diagnosis.

Dielectrophoresis-assembled zeolitic imidazolate framework nanoparticle-coupled resonators for highly sensitive and selective gas detection

This work reports on zeolitic imidazolate framework (ZIF)-coupled microscale resonators for highly sensitive and selective gas detection. The combination of microscale resonators and nanoscale materials simultaneously permits the benefit of larger capture area for adsorption from the resonator and enhanced surface adsorption capacity from the nanoscale ZIF structure. Dielectrophoresis (DEP) was demonstrated as a novel method for directly assembling concentrated ZIF nanoparticles on targeted regions of silicon resonant sensors. As part of the dielectrophoretic assembly process, the first ever measurements of the Clausius-Mossotti factor for ZIFs were conducted to determine optimal conditions for DEP assembly. The first ever real-time adsorption measurements of ZIFs were also performed to investigate the possibility of inherent gas selectivity. The ZIF-coupled resonators demonstrated sensitivity improvement up to 150 times over a bare silicon resonator with identical dimensions, and real-time adsorption measurements of ZIFs revealed different adsorption time constants for IPA and CO2.

3D printing of interdigitated Li-ion microbattery architectures

3D interdigitated microbattery architectures (3D-IMA) are fabricated by printing concentrated lithium oxide-based inks. The microbatteries are composed of interdigitated, high-aspect ratio cathode and anode structures. Our 3D-IMA, which exhibit high areal energy and power densities, may find potential application in autonomously powered microdevices.

Sub-pg mass sensing and measurement with an optomechanical oscillator

Mass sensing based on mechanical oscillation frequency shift in micro/nano scale mechanical oscillators is a well-known and widely used technique. Piezo-electric, electronic excitation/detection and free-space optical detection are the most common techniques used for monitoring the minute frequency shifts induced by added mass. The advent of optomechanical oscillator (OMO), enabled by strong interaction between circulating optical power and mechanical deformation in high quality factor optical microresonators, has created new possibilities for excitation and interrogation of micro/nanomechanical resonators. In particular, radiation pressure driven optomechanical oscillators (OMOs) are excellent candidates for mass detection/measurement due to their simplicity, sensitivity and all-optical operation. In an OMO, a high quality factor optical mode simultaneously serves as an efficient actuator and a sensitive probe for precise monitoring of the mechanical eigen-frequencies of the cavity structure. Here, we show the narrow linewidth of optomechanical oscillation combined with harmonic optical modulation generated by nonlinear optical transfer function, can result in sub-pg mass sensitivity in large silica microtoroid OMOs. Moreover by carefully studying the impact of mechanical mode selection, device dimensions, mass position and noise mechanisms we explore the performance limits of OMO both as a mass detector and a high resolution mass measurement system. Our analysis shows that femtogram level resolution is within reach even with relatively large OMOs.

Electrochemical enzyme-linked immunosorbent assay featuring proximal reagent generation: detection of human immunodeficiency virus antibodies in clinical samples

We describe a simple electrochemical immunoassay for human immunodeficiency virus (HIV) antibody detection that localizes capture and detection reagents in close proximity to a microelectrode. Antigenic peptides from HIV-1 gp41 or HIV-2 gp36 were covalently attached to a SU-8 substrate that also presented a template for the deposition of three-dimensional microelectrodes. The detection of HIV antibodies was achieved with an electrochemical immunoassay where an alkaline phosphatase conjugated secondary antibody reacts with p-aminophenyl phosphate (pAPP) to produce a redox-active product, p-aminophenol. The current derived from the oxidation of the reporter group increased linearly over a wide antibody concentration range (0.001-1 μg mL(-1)), with a detection limit of 1 ng mL(-1) (6.7 pM) for both HIV-1 and HIV-2. This level of sensitivity is clinically relevant, and the feasibility of this approach for clinical sample testing was also evaluated with HIV clinical patient samples, with excellent performance observed compared against a commercially available gold standard. This approach was used to develop the first electrochemical enzyme-linked immunosorbent assay (ELISA) to detect HIV in clinical samples, and excellent performance relative to a gold standard test was achieved.

Rapid detection of bacterial resistance to antibiotics using AFM cantilevers as nanomechanical sensors

The widespread misuse of drugs has increased the number of multiresistant bacteria, and this means that tools that can rapidly detect and characterize bacterial response to antibiotics are much needed in the management of infections. Various techniques, such as the resazurin-reduction assays, the mycobacterial growth indicator tube or polymerase chain reaction-based methods, have been used to investigate bacterial metabolism and its response to drugs. However, many are relatively expensive or unable to distinguish between living and dead bacteria. Here we show that the fluctuations of highly sensitive atomic force microscope cantilevers can be used to detect low concentrations of bacteria, characterize their metabolism and quantitatively screen (within minutes) their response to antibiotics. We applied this methodology to Escherichia coli and Staphylococcus aureus, showing that live bacteria produced larger cantilever fluctuations than bacteria exposed to antibiotics. Our preliminary experiments suggest that the fluctuation is associated with bacterial metabolism.

Biosensors based on nanomechanical systems

The advances in micro- and nanofabrication technologies enable the preparation of increasingly smaller mechanical transducers capable of detecting the forces, motion, mechanical properties and masses that emerge in biomolecular interactions and fundamental biological processes. Thus, biosensors based on nanomechanical systems have gained considerable relevance in the last decade. This review provides insight into the mechanical phenomena that occur in suspended mechanical structures when either biological adsorption or interactions take place on their surface. This review guides the reader through the parameters that change as a consequence of biomolecular adsorption: mass, surface stress, effective Young's modulus and viscoelasticity. The mathematical background needed to correctly interpret the output signals from nanomechanical biosensors is also outlined here. Other practical issues reviewed are the immobilization of biomolecular receptors on the surface of nanomechanical systems and methods to attain that in large arrays of sensors. We then describe some relevant realizations of biosensor devices based on nanomechanical systems that harness some of the mechanical effects cited above. We finally discuss the intrinsic detection limits of the devices and the limitation that arises from non-specific adsorption.

Biosensors in the small scale: methods and technology trends

This study presents a review on biosensors with an emphasis on recent developments in the field. A brief history accompanied by a detailed description of the biosensor concepts is followed by rising trends observed in contemporary micro- and nanoscale biosensors. Performance metrics to quantify and compare different detection mechanisms are presented. A comprehensive analysis on various types and subtypes of biosensors are given. The fields of interest within the scope of this review are label-free electrical, mechanical and optical biosensors as well as other emerging and popular technologies. Especially, the latter half of the last decade is reviewed for the types, methods and results of the most prominently researched detection mechanisms. Tables are provided for comparison of various competing technologies in the literature. The conclusion part summarises the noteworthy advantages and disadvantages of all biosensors reviewed in this study. Furthermore, future directions that the micro- and nanoscale biosensing technologies are expected to take are provided along with the immediate outlook.

Micromechanical PDGF recognition via lab-on-a-disc aptasensor arrays

A plug-and-play CD-like platform is used to perform a statistical detection of platelet derived growth factor (PDGF) proteins through aptamer-based surface functionalization of multiple microcantilever arrays. When PDGF proteins bind to aptamer coatings, the cantilevers deflect. The deflection response is monitored by optical read-out units from a commercial DVD-ROM device. We report on the use of an improved sensing platform which facilitates measurements under continuous liquid flow and with temperature control. Also, the mechanical wobbling of the DVD-ROM platform has been minimized and the scanning system has been optimized in order to detect cantilever deflections in liquid with nanometer scale resolution. The capability of the sensing platform is demonstrated by detection of clinically relevant concentrations of PDGF proteins. We present statistical measurements on 100 microcantilevers at different concentrations of PDGF, ranging from 10 nM to 400 nM. Hereby it is possible to reliably characterize the averaged mechanical response of cantilevers as a function of protein concentration.

Optomechanical and photothermal interactions in suspended photonic crystal membranes

We present here an optomechanical system fabricated with novel stress management techniques that allow us to suspend an ultrathin defect-free silicon photonic-crystal membrane above a Silicon-on-Insulator (SOI) substrate with a gap that is tunable to below 200 nm. Our devices are able to generate strong attractive and repulsive optical forces over a large surface area with simple in- and out- coupling and feature the strongest repulsive optomechanical coupling in any geometry to date (gOM/2π ≈65 GHz/nm). The interplay between the optomechanical and photo-thermal-mechanical dynamics is explored, and the latter is used to achieve cooling and amplification of the mechanical mode, demonstrating that our platform is well-suited for potential applications in low-power mass, force, and refractive-index sensing as well as optomechanical accelerometry.

A micro-cantilever sensor chip based on contact angle analysis for a label-free troponin I immunoassay

Cantilever sensors have been extensively explored as a promising technique for real-time and label-free analyses in biological systems. A major sensing principle utilized by state-of-the-art cantilever sensors is based on analyte-induced surface stress changes, which result in static bending of a cantilever. The sensor performance, however, suffers from the intrinsically small change in surface stress induced by analytes, especially for molecular recognition such as antigen-antibody binding. Through the contact angle change on a tailored solid surface, it is possible to convert a tiny surface stress into a capillary force-a much larger physical quantity needed for a practical sensor application. In this work, a micro-cantilever sensor based on contact angle analysis (CAMCS) was proposed to effectively enhance the sensitivity of a sensor in proportion to the square of the length to thickness ratio of the cantilever structure. CAMCS chips were fabricated using a standard complementary-metal-oxide-semiconductor (CMOS) process to demonstrate a 1250-fold enhancement in the sensitivity of surface stress to bioanalyte adsorption using a piezoresistive sensing method. A real-time and label-free troponin I (cTnI) immunoassay, which is now widely used in clinics and considered a gold standard for the early diagnosis and prognosis of cardiovascular disease, was performed to demonstrate cTnI detection levels as low as 1 pg mL(-1). The short detection time of this assay was within several minutes, which matches the detection time of commercially available instruments that are based on fluorescence-labeling techniques.

An energy-efficient readout circuit for resonant sensors based on ring-down measurement

This paper presents an energy-efficient readout circuit for resonant sensors that operates based on a transient measurement method. The resonant sensor is driven at a frequency close to its resonance frequency by an excitation source that can be intermittently disconnected, causing the sensor to oscillate at its resonance frequency with exponentially decaying amplitude. By counting the zero crossings of this ring-down response, the interface circuit can detect the resonance frequency. In contrast with oscillator-based readout, the presented readout circuit is readily able to detect quality factor (Q) of the resonator from the envelope of the ring-down response, and can be used even in the presence of large parasitic capacitors. A prototype of the readout circuit has been integrated in 0.35 μm CMOS technology, and consumes only 36 μA from a 3.3 V supply during a measurement time of 2 ms. The resonance frequency and quality factor of a micro-machined SiN resonator obtained using this prototype are in good agreement with results obtained using impedance analysis. Furthermore, a clear transient response is observed to ethanol flow using the presented readout, demonstrating the use of this technique in sensing applications.