Nanomechanical Sensing for Mass Flow Control in Nanowire-Based Open Nanofluidic Systems.

Open nanofluidic systems, where liquids flow along the outer surface of nanoscale structures, provide otherwise unfeasible capabilities for extremely miniaturized liquid handling applications. A critical step toward fully functional applications is to obtain quantitative mass flow control. We demonstrate the application of nanomechanical sensing for this purpose by integrating voltage-driven liquid flow along nanowire open channels with mass detection based on flexural resonators. This approach is validated by assembling the nanowires with microcantilever resonators, enabling high-precision control of larger flows, and by using the nanowires as resonators themselves, allowing extremely small liquid volume handling. Both implementations are demonstrated by characterizing voltage-driven flow of ionic liquids along the surface of the nanowires. We find a voltage range where mass flow rate follows a nonlinear monotonic increase, establishing a steady flow regime for which we show mass flow control at rates from below 1 ag/s to above 100 fg/s and precise liquid handling down to the zeptoliter scale. The observed behavior of mass flow rate is consistent with a voltage-induced transition from static wetting to dynamic spreading as the mechanism underlying liquid transport along the nanowires.

Square Membrane Resonators Supporting Degenerate Modes of Vibration for High-Throughput Mass Spectrometry of Single Bacterial Cells.

In nanomechanical mass spectrometry, sensing devices are commonly placed in the vacuum environment and a stream of analytes is directed toward the sensor surface for measurement. Beam structures, such as double-clamped nanobeams and nanocantilevers, are commonly used due to their low inertial mass and the simplicity of the analytical models for mass extraction. The drawback of such structures is their low capture areas, compromising the capture efficiency and throughput of this technique. Bi-axisymmetric resonators, such as ultrathin square or circular membranes, arise as an optimal geometry to maximize capture efficiency while minimizing the device inertial mass. However, these structures present degenerate mechanical modes, whose frequency perturbations upon analyte adsorption are not well described by commonly used models. Furthermore, prior knowledge of the vibration mode shapes of the sensor is crucial for the correct calculation of the analyte's mass, and the mode shape of degenerate modes may change significantly after every adsorption event. In this work, we present an accurate analytical theory to describe the effect of mass adsorption on the degenerate modes of square membrane resonators and propose two different methods based on the new theory to update the vibration mode shapes after every adsorption event. Finally, we illustrate the problem experimentally obtaining the mass and adsorption position of individual Escherichia coli K-12 bacterial cells on commercial square silicon nitride membranes fabricated with very small tolerances.

High-throughput determination of dry mass of single bacterial cells by ultrathin membrane resonators.

How bacteria are able to maintain their size remains an open question. Techniques that can measure the biomass (dry mass) of single cells with high precision and high-throughput are demanded to elucidate this question. Here, we present a technological approach that combines the transport, guiding and focusing of individual bacteria from solution to the surface of an ultrathin silicon nitride membrane resonator in vacuum. The resonance frequencies of the membrane undergo abrupt variations at the instants where single cells land on the membrane surface. The resonator design displays a quasi-symmetric rectangular shape with an extraordinary capture area of 0.14 mm2, while maintaining a high mass resolution of 0.7 fg (1 fg = 10-15 g) to precisely resolve the dry mass of single cells. The small rectangularity of the membrane provides unprecedented frequency density of vibration modes that enables to retrieve the mass of individual cells with high accuracy by specially developed inverse problem theory. We apply this approach for profiling the dry mass distribution in Staphylococcus epidermidis and Escherichia coli cells. The technique allows the determination of the dry mass of single bacterial cells with an accuracy of about 1% at an unparalleled throughput of 20 cells/min. Finally, we revisit Koch & Schaechter model developed during 60 s to assess the intrinsic sources of stochasticity that originate cell size heterogeneity in steady-state populations. The results reveal the importance of mass resolution to correctly describe these mechanisms.

Publisher Correction: Optomechanical detection of vibration modes of a single bacterium.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Optomechanical detection of vibration modes of a single bacterium.

Low-frequency vibration modes of biological particles, such as proteins, viruses and bacteria, involve coherent collective vibrations at frequencies in the terahertz and gigahertz domains. These vibration modes carry information on their structure and mechanical properties, which are good indicators of their biological state. In this work, we harnessed a particular regime in the physics of coupled mechanical resonators to directly measure these low-frequency mechanical resonances of a single bacterium. We deposit the bacterium on the surface of an ultrahigh frequency optomechanical disk resonator in ambient conditions. The vibration modes of the disk and bacterium hybridize when their associated frequencies are similar. We developed a general theoretical framework to describe this coupling, which allows us to retrieve the eigenfrequencies and mechanical loss of the bacterium low-frequency vibration modes (quality factor). Additionally, we analysed the effect of hydration on these vibrational modes. This work demonstrates that ultrahigh frequency optomechanical resonators can be used for vibrational spectrometry with the unique capability to obtain information on single biological entities.

Development of a methodology for reversible chemical modification of silicon surfaces with application in nanomechanical biosensors.

Hypervalent tellurium compounds have a particular reactivity towards thiol compounds which are related to their biological properties. In this work, this property was assembled to tellurium-functionalized surfaces. These compounds were used as linkers in the immobilization process of thiolated biomolecules (such as DNA) on microcantilever surfaces. The telluride derivatives acted as reversible binding agents due to their redox properties, providing the regeneration of microcantilever surfaces and allowing their reuse for further biomolecules immobilizations, recycling the functional surface. Initially, we started from the synthesis of 4-((3-((4-methoxyphenyl) tellanyl) phenyl) amino)-4-oxobutanoic acid, a new compound, which was immobilized on a silicon surface. In nanomechanical systems, the detection involved a hybridization study of thiolated DNA sequences. Fluorescence microscopy technique was used to confirm the immobilization and removal of the telluride-DNA system and provided revealing results about the potentiality of applying redox properties to chalcogen derivatives at surfaces.

Nanomechanical Plasmon Spectroscopy of Single Gold Nanoparticles.

We experimentally demonstrate the effect of the localized surface plasmon resonance (LSPR) of a single gold nanoparticle (AuNP) of 100 nm in diameter on the mechanical resonance frequency of a free-standing silicon nitride membrane by means of optomechanical transduction. We discover that a key effect to explain the coupling in these systems is the extinction cross section enhancement due to the excitation of the LSPR at selected wavelengths. In order to validate this coupling, we have developed a fixed wavelength interferometric readout system with an integrated tunable laser source, which allows us to perform the first experimental demonstration of nanomechanical spectroscopy of deposited AuNPs onto the membrane, discerning in between single particles and dimers by the mechanical frequency shift. We have also introduced three-axis mechanical scanners with nanometer-scale resolution in our experimental setup to selectively study single nanoparticles or small clusters. Whereas the single particles are polarization-insensitive, the gold dimers have a clearly defined polarization angle dependency as expected by theory. Finally, we found an unexpected long-distance (∼200 nm) coupling of the LSPR of separated AuNPs coming out from the guided light by the silicon nitride membrane.

Spatially Multiplexed Micro-Spectrophotometry in Bright Field Mode for Thin Film Characterization.

Thickness characterization of thin films is of primary importance in a variety of nanotechnology applications, either in the semiconductor industry, quality control in nanofabrication processes or engineering of nanoelectromechanical systems (NEMS) because small thickness variability can strongly compromise the device performance. Here, we present an alternative optical method in bright field mode called Spatially Multiplexed Micro-Spectrophotometry that allows rapid and non-destructive characterization of thin films over areas of mm² and with 1 µm of lateral resolution. We demonstrate an accuracy of 0.1% in the thickness characterization through measurements performed on four microcantilevers that expand an area of 1.8 mm² in one minute of analysis time. The measured thickness variation in the range of few tens of nm translates into a mechanical variability that produces an error of up to 2% in the response of the studied devices when they are used to measure surface stress variations.

Highly sensitive measurement of liquid density in air using suspended microcapillary resonators.

We report the use of commercially available glass microcapillaries as micromechanical resonators for real-time monitoring of the mass density of a liquid that flows through the capillary. The vibration of a suspended region of the microcapillary is optically detected by measuring the forward scattering of a laser beam. The resonance frequency of the liquid filled microcapillary is measured for liquid binary mixtures of ethanol in water, glycerol in water and Triton in ethanol. The method achieves a detection limit in an air environment of 50 µg/mL that is only five times higher than that obtained with state-of-the-art suspended microchannel resonators encapsulated in vacuum. The method opens the door to novel advances for miniaturized total analysis systems based on microcapillaries with the add-on of mechanical transduction for sensing the rheological properties of the analyzed fluids without the need for vacuum encapsulation of the resonators.

Hydration induced stress on DNA monolayers grafted on microcantilevers.

Surface tethered single-stranded DNA films are relevant biorecognition layers for oligonucleotide sequence identification. Also, hydration induced effects on these films have proven useful for the nanomechanical detection of DNA hybridization. Here, we apply nanomechanical sensors and atomic force microscopy to characterize in air and upon varying relative humidity conditions the swelling and deswelling of grafted single stranded and double stranded DNA films. The combination of these techniques validates a two-step hybridization process, where complementary strands first bind to the surface tethered single stranded DNA probes and then slowly proceed to a fully zipped configuration. Our results also demonstrate that, despite the slow hybridization kinetics observed for grafted DNA onto microcantilever surfaces, ex situ sequence identification does not require hybridization times typically longer than 1 h, while quantification is a major challenge.

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.