Thomas P. Burg

Weighing of biomolecules, single cells and single nanoparticles in fluid

Nanomechanical resonators enable the measurement of mass with extraordinary sensitivity. Previously, samples as light as 7 zeptograms (1 zg  =  10-21 g) have been weighed in vacuum, and proton-level resolution seems to be within reach. Resolving small mass changes requires the resonator to be light and to ring at a very pure tone—that is, with a high quality factor. In solution, viscosity severely degrades both of these characteristics, thus preventing many applications in nanotechnology and the life sciences where fluid is required. Although the resonant structure can be designed to minimize viscous loss, resolution is still substantially degraded when compared to measurements made in air or vacuum. An entirely different approach eliminates viscous damping by placing the solution inside a hollow resonator that is surrounded by vacuum. Here we demonstrate that suspended microchannel resonators can weigh single nanoparticles, single bacterial cells and sub-monolayers of adsorbed proteins in water with sub-femtogram resolution (1 Hz bandwidth). Central to these results is our observation that viscous loss due to the fluid is negligible compared to the intrinsic damping of our silicon crystal resonator. The combination of the low resonator mass (100 ng) and high quality factor (15,000) enables an improvement in mass resolution of six orders of magnitude over a high-end commercial quartz crystal microbalance. This gives access to intriguing applications, such as mass-based flow cytometry, the direct detection of pathogens, or the non-optical sizing and mass density measurement of colloidal particles.

Suspended microchannel resonators for biomolecular detection

We present a resonant mass sensor for specific biomolecular detection in a subnanoliter fluid volume. The sensing principle is based on measuring shifts in resonance frequency of a suspended microfluidic channel upon accumulation of molecules on the inside walls of the device. Confining the fluid to the inside of a hollow cantilever enables direct integration with conventional microfluidic systems, significantly increases sensitivity by eliminating high damping and viscous drag, and allows the resonator to be actuated by electrostatic forces. Fluid density measurements reveal a mass resolution of 10−17 g/μm2 in a 4 mHz–4 Hz bandwidth. To demonstrate biomolecular detection, we present real-time measurements of the specific binding between avidin and biotinylated bovine serum albumin. Based on these measurements, we expect that changes in surface mass loading on the order of 10−19 g/μm2 can be detected in an optimized system.

Orientation-selective aVLSI spiking neurons

We describe a programmable multi-chip VLSI neuronal system that can be used for exploring spike-based information processing models. The system consists of a silicon retina, a PIC microcontroller, and a transceiver chip whose integrate-and-fire neurons are connected in a soft winner-take-all architecture. The circuit on this multi-neuron chip approximates a cortical microcircuit. The neurons can be configured for different computational properties by the virtual connections of a selected set of pixels on the silicon retina. The virtual wiring between the different chips is effected by an event-driven communication protocol that uses asynchronous digital pulses, similar to spikes in a neuronal system. We used the multi-chip spike-based system to synthesize orientation-tuned neurons using both a feedforward model and a feedback model. The performance of our analog hardware spiking model matched the experimental observations and digital simulations of continuous-valued neurons. The multi-chip VLSI system has advantages over computer neuronal models in that it is real-time, and the computational time does not scale with the size of the neuronal network.

Measuring the mass, density, and size of particles and cells using a suspended microchannel resonator

We demonstrate the measurement of mass, density, and size of cells and nanoparticles using suspended microchannel resonators. The masses of individual particles are quantified as transient frequency shifts, while the particles transit a microfluidic channel embedded in the resonating cantilever. Mass histograms resulting from these data reveal the distribution of a population of heterogeneously sized particles. Particle density is inferred from measurements made in different carrier fluids since the frequency shift for a particle is proportional to the mass difference relative to the displaced solution. We have characterized the density of polystyrene particles, Escherichia coli, and human red blood cells with a resolution down to 10−4g∕cm3.

Vacuum-Packaged Suspended Microchannel Resonant Mass Sensor for Biomolecular Detection

There is a great need in experimental biology for tools to study interactions between biological molecules and to profile expression levels of large numbers of proteins. This paper describes the fabrication, packaging and testing of a resonant mass sensor for the detection of biomolecules in a microfluidic format. The transducer employs a suspended microchannel as the resonating element, thereby avoiding the problems of damping and viscous drag that normally degrade the sensitivity of resonant sensors in liquid. Our device differs from a vibrating tube densitometer in that the channel is very thin, which enables the detection of molecules that bind to the channel walls; this provides a path to specificity via molecular recognition by immobilized receptors. The fabrication is based on a sacrificial polysilicon process with low-stress low-pressure chemical-vapor deposited (LPCVD) silicon nitride as the structural material, and the resonator is vacuum packaged on the wafer scale using glass frit bonding. Packaged resonators exhibit a sensitivity of 0.8 ppm/(ngmiddotcm2) and a mechanical quality factor of up to 700. To the best of our knowledge, this quality factor is among the highest so far reported for resonant sensors with comparable surface mass sensitivity in liquid

Toward attogram mass measurements in solution with suspended nanochannel resonators.

Using suspended nanochannel resonators (SNRs), we demonstrate measurements of mass in solution with a resolution of 27 ag in a 1 kHz bandwidth, which represents a 100-fold improvement over existing suspended microchannel resonators and, to our knowledge, is the most precise mass measurement in liquid today. The SNR consists of a cantilever that is 50 microm long, 10 microm wide, and 1.3 microm thick, with an embedded nanochannel that is 2 microm wide and 700 nm tall. The SNR has a resonance frequency near 630 kHz and exhibits a quality factor of approximately 8000 when dry and when filled with water. In addition, we introduce a new method that uses centrifugal force caused by vibration of the cantilever to trap particles at the free end. This approach eliminates the intrinsic position dependent error of the SNR and also improves the mass resolution by increasing the averaging time for each particle.

Microfabricated mechanical biosensor with inherently differential readout

We report measurements with a micromachined mechanical biosensor that inherently suppresses background effects by producing a differential signal with respect to a reference. The sensor comprises two adjacent cantilevers with interdigitated fingers between them that allow interferometric detection of the differential, i.e., relative bending. We demonstrate that differential detection can efficiently suppress unspecific chemical effects that result in cantilever bending. We show that the differential deflection noise is up to an order of magnitude lower than the absolute deflection noise in the low-frequency range of 0.0003–1 Hz, where many types of biologically relevant reactions occur. We also demonstrate the sensor’s applicability to biological receptor–ligand systems by reporting experimental results on direct differential detection of biotin–streptavidin binding.

Fabrication and characterization of a micromechanical sensor for differential detection of nanoscale motions

We have micromachined a mechanical sensor that uses interferometry to detect the differential and absolute deflections of two adjacent cantilevers. The overall geometry of the device allows simple fluidic delivery to each cantilever to immobilize molecules for biological and chemical detection. We show that differential sensing is 50 times less affected by ambient temperature changes than the absolute, thus enabling a more reliable differentiation between specific cantilever bending and background effects. We describe the fabrication process and show results related to the dynamic characterization of the device as a differential sensor. The root-mean-squared (r.m.s.) sensor noise in water and air is /spl sim/1 nm over the frequency range of 0.4-40 Hz. We also find that in air, the deflection resolution is limited only by the cantilevers thermomechanical noise level of 0.008 /spl Aring//Hz/sup 1/2/ over the frequency range of 40-1000 Hz.

Energy dissipation in microfluidic beam resonators.

The fluid–structure interaction of resonating microcantilevers immersed in fluid has been widely studied and is a cornerstone in nanomechanical sensor development. In many applications, fluid damping imposes severe limitations by strongly degrading the signal-to-noise ratio of measurements. Recently, Burg et al . ( Nature , vol. 446, 2007, pp. 1066–1069) proposed an alternative type of microcantilever device whereby a microfluidic channel was embedded inside the cantilever with vacuum outside. Remarkably, it was observed that energy dissipation in these systems was almost identical when air or liquid was passed through the channel and was 4 orders of magnitude lower than that in conventional microcantilever systems. Here, we study the fluid dynamics of these devices and present a rigorous theoretical model corroborated by experimental measurements to explain these observations. In so doing, we elucidate the dominant physical mechanisms giving rise to the unique features of these devices. Significantly, it is found that energy dissipation is not a monotonic function of fluid viscosity, but exhibits oscillatory behaviour, as fluid viscosity is increased/decreased. In the regime of low viscosity, inertia dominates the fluid motion inside the cantilever, resulting in thin viscous boundary layers – this leads to an increase in energy dissipation with increasing viscosity. In the high-viscosity regime, the boundary layers on all surfaces merge, leading to a decrease in dissipation with increasing viscosity. Effects of fluid compressibility also become significant in this latter regime and lead to rich flow behaviour. A direct consequence of these findings is that miniaturization does not necessarily result in degradation in the quality factor, which may indeed be enhanced. This highly desirable feature is unprecedented in current nanomechanical devices and permits direct miniaturization to enhance sensitivity to environmental changes, such as mass variations, in liquid.

Suspended microchannel resonators for ultralow volume universal detection.

Universal detectors that maintain high sensitivity as the detection volume is reduced to the subnanoliter scale can enhance the utility of miniaturized total analysis systems (mu-TAS). Here the unique scaling properties of the suspended microchannel resonator (SMR) are exploited to show universal detection in a 10 pL analysis volume with a density detection limit of approximately 1 microg/cm (3) (10 Hz bandwidth) and a dynamic range of 6 decades. Analytes with low UV extinction coefficients such as polyethylene glycol (PEG) 8 kDa, glucose, and glycine are measured with molar detection limits of 0.66, 13.5, and 31.6 microM, respectively. To demonstrate the potential for real-time monitoring, gel filtration chromatography was used to separate different molecular weights of PEG as the SMR acquired a chromatogram by measuring the eluate density. This work suggests that the SMR could offer a simple and sensitive universal detector for various separation systems from liquid chromatography to capillary electrophoresis. Moreover, since the SMR is itself a microfluidic channel, it can be directly integrated into mu-TAS without compromising overall performance.