Alexis Vlandas

Studying biological tissue with fluorescence lifetime imaging: microscopy, endoscopy, and complex decay profiles

We have applied fluorescence lifetime imaging (FLIM) to the autofluorescence of different kinds of biological tissue in vitro, including animal tissue sections and knee joints as well as human teeth, obtaining two-dimensional maps with functional contrast. We find that fluorescence decay profiles of biological tissue are well described by the stretched exponential function (StrEF), which can represent the complex nature of tissue. The StrEF yields a continuous distribution of fluorescence lifetimes, which can be extracted with an inverse Laplace transformation, and additional information is provided by the width of the distribution. Our experimental results from FLIM microscopy in combination with the StrEF analysis indicate that this technique is ready for clinical deployment, including portability that is through the use of a compact picosecond diode laser as the excitation source. The results obtained with our FLIM endoscope successfully demonstrated the viability of this modality, though they need further optimization. We expect a custom-designed endoscope with optimized illumination and detection efficiencies to provide significantly improved performance.

Lysozyme detection on aptamer functionalized graphene-coated SPR interfaces.

The paper reports on a surface plasmon resonance (SPR)-based approach for the sensitive and selective detection of lysozyme. The SPR sensor consists of a 50 nm gold film coated with a thin film of reduced graphene oxide (rGO) functionalized with anti-lysozyme DNA aptamer. The SPR chip coating with rGO matrix was achieved through electrophoretic deposition of graphene oxide (GO) at 150 V. Electrophoretic deposition resulted in partial reduction of GO to rGO with a thickness depending on the deposition time. For very short time pulses of 20 s, the resulting rGO film had a thickness of several nanometers and was appropriate for SPR sensing. The utility of the graphene-based SPR sensor for the selective and sensitive detection of proteins was demonstrated using lysozyme as model protein. Functionalization of rGO matrix with anti-lysozyme DNA aptamer through π-stacking interactions allowed selective SPR detection of lysozyme. The graphene-based SPR biosensor provides a means for the label-free, concentration-dependent and selective detection of lysozymes with a detection limit of 0.5 nM.

Enzyme-Free Sugar Sensing in Microfluidic Channels with an Affinity-Based Single-Wall Carbon Nanotube Sensor

We present a novel nonenzymatic carbon nanotube sensor integrated in a microfluidic channel for the detection of sugars. The sensor is assembled as a liquid-gated field-effect transistor, with the transistor channel composed of 1 to 10 nanotubes, which are controllably functionalized with boronic acid receptors. The devices show sensitivity to glucose in a concentration range of 5 to 30 mM. Furthermore, by controlling the type of nanotube-receptor coupling (as covalent or noncovalent) and by deploying a sensitive impedance-based detection technique, we corroborate in detail the transduction mechanism of our affinity-based sensor. In the case of covalent coupling, charge carrier scattering along the nanotubes is the dominant mechanism. While in the noncovalent case, surface charge effects dominate. The identification of the mechanism along with the tunability of the chemical coupling and the cost-effective integration in microchannels constitute a solid basis for the entry of nanotube-based sensors in lab-on-a-chip applications.

Label-Free Detection of Few Copies of DNA with Carbon Nanotube Impedance Biosensors†

Keywords: carbon nanotubes ; DNA assays ; electrochemical functionalization ; impedance biosensors ; nanobiosensors ; Field-Effect Transistors ; Silicon Nanowires ; Hybridization ; Sensors ; Diagnostics ; Dependence Reference EPFL-ARTICLE-171548doi:10.1002/anie.201006806View record in Web of Science Record created on 2011-12-16, modified on 2017-05-12

Site-Specific Self-Assembled Liquid-Gated ZnO Nanowire Transistors for Sensing Applications

A scalable bottom-up solution-based approach for the site-specific realization of ZnO nanowire (ZnO-NW)-based field-effect transistors for sensing applications in liquids is reported. The nanowires are grown across predefined electrodes patterned by photolithography. Site specificity is attained by the use of nanoparticles acting as seeds. Using integrated on-chip microchannels and microfabricated gate electrodes, electrochemically gated ZnO-NW network transistors functioning in liquids are demonstrated. The optimized devices are rendered sensitive to pH through chemical functionalization. The unique combination of the sensitivity, site specificity, scalability, and cost effectiveness of the technique opens up avenues for the routine realization of one-dimensional nanostructure-based chemical and biosensors for analytical and diagnostic applications.

Electrical impedance sensor for quantitative monitoring of infection processes on HCT-8 cells by the waterborne parasite Cryptosporidium.

Cryptosporidium is the main origin of worldwide waterborne epidemic outbreaks caused by protozoan parasites. Its resilience to water chemical treatments and the absence of therapy led to consider it as a reference pathogen to assess water quality and as a possible bioterrorism agent. We here show that an electrical impedance-based device is able to get insights on Cryptosporidium development on a cell culture and to quantify sample infectivity. HCT-8 cells were grown to confluency on Interdigitated Microelectrode Arrays (IMAs) during 76h and then infected by Cryptosporidium parvum during 60h. The impedimetric response was measured at frequencies ranging from 100Hz to 1MHz and a 7min sampling period. As the infection progresses the impedance signal shows a reproducible distinct succession of peaks at 12h post infection (PI), 23h PI and 31h PI and local minima at 9h PI, 19h PI and 28h PI. An equivalent circuit modeling-based approach indicates that these features are mostly originated from paracellular pathway modifications due to host-parasite interactions. Furthermore, our data present for the first time a real-time monitoring of early parasitic stage development with alternating zoite and meront predominances, observed respectively at peaks and local minima in the impedimetric signal. Finally, by quantifying the magnitude of the impedimetric response, we demonstrate this device can also be used as an infectivity sensor as early as 12h PI thus being at least 6 times faster than other state of the art techniques.

Ab initio infrared vibrational modes for neutral and charged small fullerenes (C20, C24, C26, C28, C30 and C60).

We calculate the infrared (IR) absorption spectra using DFT B3LYP(6–311G) for a range of small closed-cage fullerenes, Cn, n=20, 24, 26, 28, 30 and 60, in both neutral and multiple positive and negative charge states. The results are of use, notably, for direct comparison with observed IR absorption in the interstellar medium. Frequencies fall typically into two ranges, with C−C stretch modes around 1100–1500 cm−1 (6.7–9.1 μm) and fullerene-specific radial motion associated with under-coordinated carbon at pentagonal sites in the range 600–800 cm−1 (12.5–16.7 μm). Notably, negatively charged fullerenes show significantly stronger absorption intensities than neutral species. The results suggest that small cage fullerenes, and notably metallic endofullerenes, may be responsible for many of the unassigned interstellar IR spectral lines. This article is part of the themed issue ‘Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene’.

Band-Gap Modification Induced in HgTe by Dimensional Constraint in Carbon Nanotubes: Effect of Nanotube Diameter on Microstructure

A new tubular form of HgTe grown in narrow single walled carbon nanotubes is described with Hg and Te in reduced coordination. Two unique projections obtained by HRTEM from two separate crystal fragments enabled reconstruction of the atomic arrangement of the new form. DFT confirmed the stability of the new structure and that it has a modified band gap, transforming HgTe from a semimetal to a semiconductor (band gap +1.3eV). HRTEM shows that as the nanotube diameter increases, the new form is no longer obtained and for diameters of 1.6-2 nm, disordered HgTe is obtained, for diameters >2 nm, sphalerite HgTe is obtained.

Superhydrophobic surfaces’ influence on streaming current based energy harvester

The purpose of this paper is to report the design, fabrication and characterization of silicon-based microfluidic channels with superhydrophobic walls for energy harvesting. We present the fabrication step of silicon based streaming current energy harvester and the nanostructuration of the microchannel walls. We characterize the superhydrophobic properties of the surface in a closed system. Our preliminary results on the electrical characterization of the device show a 43% increase of power harvested with our superhydrophobic surface compared to a planar hydrophobic surface.

Spatiotemporal control of DNA-based chemical reaction network via electrochemical activation in microfluidics

In recent years, DNA computing frameworks have been developed to create dynamical systems which can be used for information processing. These emerging synthetic biochemistry tools can be leveraged to gain a better understanding of fundamental biology but can also be implemented in biosensors and unconventional computing. Most of the efforts so far have focused on changing the topologies of DNA molecular networks or scaling them up. Several issues have thus received little attention and remain to be solved to turn them into real life technologies. In particular, the ability to easily interact in real-time with them is a key requirement. The previous attempts to achieve this aim have used microfluidic approaches, such as valves, which are cumbersome. We show that electrochemical triggering using DNA-grafted micro-fabricated gold electrodes can be used to give instructions to these molecular systems. We demonstrate how this approach can be used to release at specific times and locations DNA- based instructions. In particular, we trigger reaction-diffusion autocatalytic fronts in microfluidic channels. While limited by the stability of the Au-S bond, this easy to implement, versatile and scalable technique can be used in any biology laboratory to provide new ways to interact with any DNA-based computing framework.