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Papers by Leonardo Lesser-Rojas

Journal of Applied Physics, 2008

Metallic subwavelength apertures can be used in epi-illumination fluorescence to achieve focal vo... more Metallic subwavelength apertures can be used in epi-illumination fluorescence to achieve focal volume confinement. Because of the near field components inherent to small metallic structures, observation volumes are formed that are much smaller than the conventional diffraction limited volume attainable by high numerical aperture far field optics (circa a femtoliter). Observation volumes in the range of 10−4fl have been reported previously. Such apertures can be used for single-molecule detection at relatively high concentrations (up to 20μM) of fluorophores. Here, we present a novel fabrication of metallic subwavelength apertures in the visible range. Using a new electron beam lithography process, uniform arrays of such apertures can be manufactured efficiently in large numbers with diameters in the range of 60–100nm. The apertures were characterized by scanning electron microscopy, optical microscopy, focused ion beam cross sections/transmission electron microscopy, and fluorescenc...

Micromachines

Blood is a complex sample comprised mostly of plasma, red blood cells (RBCs), and other cells who... more Blood is a complex sample comprised mostly of plasma, red blood cells (RBCs), and other cells whose concentrations correlate to physiological or pathological health conditions. There are also many blood-circulating biomarkers, such as circulating tumor cells (CTCs) and various pathogens, that can be used as measurands to diagnose certain diseases. Microfluidic devices are attractive analytical tools for separating blood components in point-of-care (POC) applications. These platforms have the potential advantage of, among other features, being compact and portable. These features can eventually be exploited in clinics and rapid tests performed in households and low-income scenarios. Microfluidic systems have the added benefit of only needing small volumes of blood drawn from patients (from nanoliters to milliliters) while integrating (within the devices) the steps required before detecting analytes. Hence, these systems will reduce the associated costs of purifying blood components o...

Molecules

Amyloid oligomeric species, formed during misfolding processes, are believed to play a major role... more Amyloid oligomeric species, formed during misfolding processes, are believed to play a major role in neurodegenerative and metabolic diseases. Deepening the knowledge about the structure of amyloid intermediates and their aggregation pathways is essential in understanding the underlying mechanisms of misfolding and cytotoxicity. However, structural investigations are challenging due to the low abundance and heterogeneity of those metastable intermediate species. Single-molecule techniques have the potential to overcome these difficulties. This review aims to report some of the recent advances and applications of vibrational spectroscopic techniques for the structural analysis of amyloid oligomers, with special focus on single-molecule studies.

This work reports a versatile platform for manipulation and sensing of biological molecules based... more This work reports a versatile platform for manipulation and sensing of biological molecules based on electrode nanogaps, which function as dielectrophoresis-enabled molecular trapping templates as well as spectroscopic and nanoelectronic detection substrates. On a chip, an array was constructed of Ti/Al nanoelectrode pairs with gap dimensions ranging from few tens of nm to <10 nm. During the molecular trapping process, ionic current measurements across the nanoelectrodes were used to detect trapping of a protein in the gap. Concurrent recording of time-dependent Raman spectra provided evidence of molecule(s) appearing in the nanogap.

Nano Letters, 2014

We report a versatile analysis platform, based on a set of nanogap electrodes, for the manipulati... more We report a versatile analysis platform, based on a set of nanogap electrodes, for the manipulation and sensing of biomolecules, as demonstrated here for low-copy number protein detection. An array of Ti nanogap electrode with sub-10 nm gap size function as templates for alternating current dielectrophoresis-based molecular trapping, hot spots for surface-enhanced Raman spectroscopy as well as electronic measurements, and fluorescence imaging. During molecular trapping, recorded Raman spectra, conductance measurements across the nanogaps, and fluorescence imaging show unambiguously the presence and characteristics of the trapped proteins. Our platform opens up a simple way for multifunctional low-concentration heterogeneous sample analysis without the need for target preconcentration.

Analytical Chemistry, 2021

Alzheimer's disease (AD) has become highly relevant in aging societies, yet the fundamental m... more Alzheimer's disease (AD) has become highly relevant in aging societies, yet the fundamental molecular basis for AD is still poorly understood. New tools to study the undergoing structural conformation changes of amyloid beta (Aβ) peptides, the pathogenic hallmark of AD, could play a crucial role in the understanding of the underlying mechanisms of misfolding and cytotoxicity of this peptide. It has been recently reported that Zn2+ interacts with Aβ and changes its aggregation pathway away from less harmful fibrillar forms to more toxic species. Here, we present a versatile platform based on a set of sub-10 nm nanogap electrodes for the manipulation and sensing of biomolecules in the physiological condition at a low copy number, where molecules are trapped via dielectrophoresis (DEP) across the nanogap, which also serves as a surface-enhanced Raman spectroscopy hotspot. In this study, we demonstrate that our electrode nanogap platform can be used to study the structural difference between Aβ40 and ZnAβ40 peptides at different aggregation stages in the physiologically relevant concentration and in solution phase. The Raman spectroscopic signatures of the DEP-captured neuropeptides prove the device to be attractive as a label-free bioanalytical tool.

Biophysical Journal, 2021

AIP Advances

A low-cost fabrication method of microfluidic devices with micrometer-sized constrictions used fo... more A low-cost fabrication method of microfluidic devices with micrometer-sized constrictions used for electrodeless dielectrophoresis (eDEP) is demonstrated here. A structure on a commercial printed circuit board (PCB) template of one-sided copper clad fiberglass-epoxy laminate was used as a molding master for polydimethylsiloxane (PDMS) soft lithography. This was achieved by printing a constriction-based microchannel pattern on glossy paper with a micrometer-scaled resolution laser printer and transferring it to the laminate's Cu face, rendering a microstructure of ∼17 μm height and various widths across tips. The Cu master's pattern was transferred to PDMS, and smooth constrictions were observed under the microscope. Following air plasma encapsulation, PDMS chips were loaded with an inactivated bacterial sample of fluorescently stained Brucella abortus vaccine strain S-19 and connected to an amplified voltage source to examine the sample's response to electric field variations. After an AC/DC electric field was applied to the bacterial solution in the microfluidic device, the combined effect of electrokinetic + hydrodynamic mechanisms that interact near the dielectric microconstrictions and exert forces to the sample was observed and later confirmed by COMSOL simulations. Our fabrication method is an alternative to be used when there is no access to advanced microfabrication facilities and opens ways for target selection and preconcentration of intracellular pathogens as well as sample preparation for metagenomics.

Biomicrofluidics, 2014

We have developed a two-step electron-beam lithography process to fabricate a tandem array of thr... more We have developed a two-step electron-beam lithography process to fabricate a tandem array of three pairs of tip-like gold nanoelectronic detectors with electrode gap size as small as 9 nm, embedded in a coplanar fashion to 60 nm deep, 100 nm wide, and up to 150 μm long nanochannels coupled to a world-micro-nanofluidic interface for easy sample introduction. Experimental tests with a sealed device using DNA-protein complexes demonstrate the coplanarity of the nanoelectrodes to the nanochannel surface. Further, this device could improve transverse current detection by correlated time-of-flight measurements of translocating samples, and serve as an autocalibrated velocimeter and nanoscale tandem Coulter counters for single molecule analysis of heterogeneous samples.