The advances in micro-and nanofabrication technologies are enabling 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 bioreceptor molecules on the surface of nanomechanical sensors and methods to attain that in large arrays of sensors. We describe then 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.2

Blood contains a range of protein biomarkers that could be used in the early detection of disease. To achieve this, however, requires sensors capable of detecting (with high reproducibility) biomarkers at concentrations one million times lower than the concentration of the other blood proteins. Here, we show that a sandwich assay that combines mechanical and optoplasmonic transduction can detect cancer biomarkers in serum at ultralow concentrations. A biomarker is first recognized by a surface-anchored antibody and then by an antibody in solution that identifies a free region of the captured biomarker. This second antibody is tethered to a gold nanoparticle that acts as a mass and plasmonic label; the two signatures are detected by means of a silicon cantilever that serves as a mechanical resonator for 'weighing' the mass of the captured nanoparticles and as an optical cavity that boosts the plasmonic signal from the nanoparticles. The capabilities of the approach are illustrated with two cancer biomarkers: the carcinoembryonic antigen and the prostate specific antigen, which are currently in clinical use for the diagnosis, monitoring and prognosis of colon and prostate cancer, respectively. A detection limit of 1 × 10(-16) g ml(-1) in serum is achieved with both biomarkers, which is at least seven orders of magnitude lower than that achieved in routine clinical practice. Moreover, the rate of false positives and false negatives at this concentration is extremely low, ∼10(-4).

The identification of species is a fundamental problem in analytical chemistry and biology. Mass spectrometers identify species by their molecular mass with extremely high sensitivity (<10−24 g). However, its application is usually limited to light analytes (<10−19 g). Here we demonstrate that by using nanomechanical resonators, heavier analytes can be identified by their mass and stiffness. The method is demonstrated with spherical gold nanoparticles and whole intact E. coli bacteria delivered by electrospray ionization to microcantilever resonators placed in low vacuum at 0.1 torr. We develop a theoretical procedure for obtaining the mass, position and stiffness of the analytes arriving the resonator from the adsorption-induced eigenfrequency jumps. These results demonstrate the enormous potential of this technology for identification of large biological complexes near their native conformation, a goal that is beyond the capabilities of conventional mass spectrometers.

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 harness a particular regime in the physics of coupled mechanical resonators to directly measure the mechanical resonances of single bacterium. We deposit the bacterium on the surface of an ultra-high frequency optomechanical disk resonator in ambient conditions. The vibration modes of the disk and bacterium hybridize when their associated frequencies are similar. We develop a general theoretical framework to describe this coupling, which allows us to retrieve the eigenfrequencies and mechanical loss of the bacterium vibration modes (Q factor). Finally, we analyse the effect of hydration on the vibrational properties of a single bacterium. 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.

There is an emerging need of nanotools able to quantify the mechanical properties of single biological entities. A promising approach is the measurement of the shifts of the resonant frequencies of ultrathin cantilevers induced by the adsorption of the studied biological systems. Here, we present a detailed theoretical analysis to calculate the resonance frequency shift induced by the mechanical stiffness of viral nanotubes. The model accounts for the high surface-to-volume ratio featured by single biological entities, the shape anisotropy and the interfacial adhesion. The model is applied to the case in which tobacco mosaic virus is randomly delivered to a silicon nitride cantilever. The theoretical framework opens the door to a novel paradigm for biological spectrometry as well as for measuring the Young's modulus of biological systems with minimal strains.I t is increasingly evident the intimate link between the mechanical properties of biological systems and its role in fundamental biological processes and disease 1 . This link spans from the molecular scale to the tissue scale. For example, the elasticity of cells has become a reliable indicator of cell transformation into cancerous or metastatic cells [2][3] . Similarly, recent reports have demonstrated the biological relevance of the mechanical properties of viruses. Viruses are able to dynamically modulate their mechanical properties in response to external forces, so as to withstand those forces or to ease cell infection 4 . For instance, in the human immunodeficiency and murine leukemia viruses, the stiffness largely decreases during the maturation process, acting as a mechanical switch for the infection process 5 . Strikingly, a single point mutation in the capsid protein of some viruses can significantly change their elasticity 6 . It is therefore fundamental the development of nanotools that enable the accurate quantification of the nanomechanical properties of single biological entities with high throughput. These tools can provide new insights on how the structural conformation, biological function, and mechanical properties of biomolecules and their hierarchical assemblies are related each other. The most prominent method to measure the mechanical properties of biological entities has been so far nanoindentation with the cantilever/tip assembly of an atomic force microscope (AFM) 7 . However, a number of challenges exist with the AFM for the quantification of the mechanical properties. Mainly, the nanoindentation curves strongly depend on the nanometer-scale geometry of the tip/sample contact, which in most of the cases cannot be controlled. Other difficulties include the contribution of the underlying substrate, the effect of adhesion, non-linear loading and the lack of accurate theoretical models.We envisage a novel biological spectrometry technique based on the measurement of several vibration modes of ultrathin micro-and nanocantilevers for the identification of adsorbed biomolecules and biological systems by two coordinates: the mass 8-11 an...

Microcantilever biosensors in the static operation mode translate molecular recognition into a surface stress signal. Surface stress is derived from the nanomechanical cantilever bending by applying Stoney's equation, derived more than one hundred years ago. This equation ignores the clamping effect on the cantilever deformation, which induces significant errors in the quantification of the biosensing response.This leads to discrepancies in the surface-stress induced by biomolecular interactions in measurements with cantilevers with different sizes and geometries. So far, more accurate solutions have been precluded by the formidable complexity of the theoretical problem that involves solving the twodimensional biharmonic equation. In this Letter, we present an accurate and simple analytical expression to quantify the response of microcantilever biosensors. The equation exhibits an excellent agreement with finite elements simulations and experiments of DNA immobilization on gold-coated microcantilevers.*These three authors contributed equally to this

Monitoring the effect of the substrate on the local surface plasmon resonance (LSPR) of metallic nanoparticles is key for deepening our understanding of light-matter interactions at the nanoscale. This coupling gives rise to shifts of the LSPR as well as changes in the scattering pattern shape. The problem requires of high-throughput techniques that present both high spatial and spectral resolution. We present here a technique, referred to as Spatially Multiplexed Micro-Spectrophotometry (SMMS), able to perform polarization-resolved spectral and spatial analysis of the scattered light over large surface areas. The SMMS technique provides three orders of magnitude faster spectroscopic analysis than conventional dark-field microspectrophotometry, with the capability for mapping the spatial distribution of the scattered light intensity with lateral resolution of 40 nm over surface areas of 0.02 mm2. We show polarization-resolved dark-field spectral analysis of hundreds of gold nanoparticles deposited on a silicon surface. The technique allows determining the effect of the substrate on the LSPR of single nanoparticles and dimers and their scattering patterns. This is applied for rapid discrimination and counting of monomers and dimers of nanoparticles. In addition, the diameter of individual nanoparticles can be rapidly assessed with 1 nm accuracy.

Curved thin sheets are ubiquitously found in nature and manmade structures from macro- to nanoscale. Within the framework of classical thin plate theory, the stiffness of thin sheets is independent of its bending state for small deflections. This assumption, however, goes against intuition. Simple experiments with a cantilever sheet made of paper show that the cantilever stiffness largely increases with small amounts of transversal curvature. We here demonstrate by using simple geometric arguments that thin sheets subject to two-dimensional bending necessarily develop internal stresses. The coupling between the internal stresses and the bending moments can increase the stiffness of the plate by several times. We develop a theory that describes the stiffness of curved thin sheets with simple equations in terms of the longitudinal and transversal curvatures. The theory predicts experimental results with a macroscopic cantilever sheet as well as numerical simulations by the finite element method. The results shed new light on plant and insect wing biomechanics and provide an easy route to engineer micro- and nanomechanical structures based on thin materials with extraordinary stiffness tunability.