10 research outputs found
Electrical Double Layer-Induced Ion Surface Accumulation for Ultrasensitive Refractive Index Sensing with Nanostructured Porous Silicon Interferometers
Herein, we provide
the first experimental evidence on the use of
electrical double layer (EDL)-induced accumulation of charged ions
(using both Na<sup>+</sup> and K<sup>+</sup> ions in water as the
model) onto a negatively charged nanostructured surface (e.g., thermally
growth SiO<sub>2</sub>)Ion Surface Accumulation, ISAas
a means of improving performance of nanostructured porous silicon
(PSi) interferometers for optical refractometric applications. Nanostructured
PSi interferometers are very promising optical platforms for refractive
index sensing due to PSi huge specific surface (hundreds of m<sup>2</sup> per gram) and low preparation cost (less than $0.01 per 8
in. silicon wafer), though they have shown poor resolution (<i>R</i>) and detection limit (DL) (on the order of 10<sup>–4</sup>–10<sup>–5</sup> RIU) compared to other plasmonic and
photonic platforms (<i>R</i> and DL on the order of 10<sup>–7</sup>–10<sup>–8</sup> RIU). This can be ascribed
to both low sensitivity and high noise floor of PSi interferometers
when bulk refractive index variation of the solution infiltrating
the nanopores either approaches or is below 10<sup>–4</sup> RIU. Electrical double layer-induced ion surface accumulation (EDL-ISA)
on oxidized PSi interferometers allows the interferometer output signal
(spectral interferogram) to be impressively amplified at bulk refractive
index variation below 10<sup>–4</sup> RIU, increasing, in turn,
sensitivity up to 2 orders of magnitude and allowing reliable measurement
of refractive index variations to be carried out with both DL and
R of 10<sup>–7</sup> RIU. This represents a 250-fold-improvement
(at least) with respect to the state-of-the-art literature on PSi
refractometers and pushes PSi interferometer performance to that of
state-of-the-art ultrasensitive photonics/plasmonics refractive index
platforms
Flexible Polydimethylsiloxane Foams Decorated with Multiwalled Carbon Nanotubes Enable Unprecedented Detection of Ultralow Strain and Pressure Coupled with a Large Working Range
Low-cost piezoresistive
strain/pressure sensors with large working
range, at the same time able to reliably detect ultralow strain (≤0.1%)
and pressure (≤1 Pa), are one of the challenges that have still
to be overcome for flexible piezoresistive materials toward personalized
health-monitoring applications. In this work, we report on unprecedented,
simultaneous detection of ultrasmall strain (0.1%, i.e., 10 μm
displacement over 10 mm) and subtle pressure (20 Pa, i.e., a force
of only 2 mN over an area of 1 cm<sup>2</sup>) in compression mode,
coupled with a large working range (i.e., up to 60% for strain6
mm in displacementand 50 kPa for pressure) using piezoresistive,
flexible three-dimensional (3D) macroporous polydimethylsiloxane
(pPDMS) foams decorated with pristine multiwalled carbon nanotubes
(CNTs). pPDMS/CNT foams with pore size up to 500 μm (i.e., twice
the size of those of commonly used foams, at least) and porosity of
77%, decorated with a nanostructured surface network of CNTs at densities
ranging from 7.5 to 37 mg/cm<sup>3</sup> are prepared using a low-cost
and scalable process, through replica molding of sacrificial sugar
templates and subsequent drop-casting of CNT ink. A thorough characterization
shows that piezoresistive properties of the foams can be finely tuned
by controlling the CNT density and reach an optimum at a CNT density
of 25 mg/cm<sup>3</sup>, for which a maximum change of the material
resistivity (e.g., ρ<sub>0</sub>/ρ<sub>50</sub> = 4 at
50% strain) is achieved under compression. Further static and dynamic
characterization of the pPDMS/CNT foams with 25 mg/cm<sup>3</sup> of
CNTs highlights that detection limits for strain and pressure are
0.03% (3 μm displacement over 10 mm) and 6 Pa (0.6 mN over an
area of 1 cm<sup>2</sup>), respectively; moreover, good stability
and limited hysteresis are apparent by cycling the foams with 255
compression–release cycles over the strain range of 0–60%,
at different strain rates up to 10 mm/min. Our results on piezoresistive,
flexible pPDMS/CNT foams pave the way toward breakthrough applications
for personalized health care, though not limited to these, which have
not been fully addressed to date with flexible strain/stress sensors
Sub-Parts Per Million NO<sub>2</sub> Chemi-Transistor Sensors Based on Composite Porous Silicon/Gold Nanostructures Prepared by Metal-Assisted Etching
Surface doping of nano/mesostructured
materials with metal nanoparticles to promote and optimize chemi-transistor
sensing performance represents the most advanced research trend in
the field of solid-state chemical sensing. In spite of the promising
results emerging from metal-doping of a number of nanostructured semiconductors,
its applicability to silicon-based chemi-transistor sensors has been
hindered so far by the difficulties in integrating the composite metal–silicon
nanostructures using the complementary metal-oxide-semiconductor (CMOS)
technology. Here we propose a facile and effective top-down method
for the high-yield fabrication of chemi-transistor sensors making
use of composite porous silicon/gold nanostructures (cSiAuNs) acting
as sensing gate. In particular, we investigate the integration of
cSiAuNs synthesized by metal-assisted etching (MAE), using gold nanoparticles
(NPs) as catalyst, in solid-state junction-field-effect transistors
(JFETs), aimed at the detection of NO<sub>2</sub> down to 100 parts
per billion (ppb). The chemi-transistor sensors, namely cSiAuJFETs,
are CMOS compatible, operate at room temperature, and are reliable,
sensitive, and fully recoverable for the detection of NO<sub>2</sub> at concentrations between 100 and 500 ppb, up to 48 h of continuous
operation
10 000-Fold Improvement in Protein Detection Using Nanostructured Porous Silicon Interferometric Aptasensors
In-field
analysis (e.g., clinical and diagnostics) using nanostructured
porous silicon (PSi) for label-free optical biosensing has been hindered
so far by insufficient sensitivity of PSi biosensors. Here we report
on a label-free PSi interferometric aptasensor able to specifically
detect tumor necrosis factor alpha (TNFα, a protein biomarker
of inflammation and sepsis) at concentration down to 3.0 nM with signal-to-noise
ratio (S/N) of 10.6 and detection limit (DL) of 200 pM. This represents
a 10 000-fold improvement with respect to direct (i.e., nonamplified)
label-free PSi biosensors and pushes PSi biosensors close to the most
sensitive optical and label-free transduction techniques, e.g., surface
plasmon resonance (SPR) for which a lowest DL of 100 pM in aptasensing
has been reported. A factor 1000 in improvement is achieved by introducing
a novel signal-processing technique for the optical readout of PSi
interferometers, namely, interferogram average over wavelength (IAW)
reflectance spectroscopy. The IAW reflectance spectroscopy is shown
to significantly improve both sensitivity and reliability of PSi biosensors
with respect to commonly used fast Fourier transform (FFT) reflectance
spectroscopy. A further factor 10 is achieved by enabling preparation
of PSi interferometers with enlarged pore sizes (up to a 3× increase
in diameter) at constant current density (i.e., constant porosity
and, in turn, constant refractive index). This method is in contrast
to standard PSi preparation where pore size is increased by increasing
etching current density (i.e., porosity), and allows tackling mass-limited
diffusion of biomolecules into the nanopores without worsening PSi
interferometer optical features
Long-Range Order in Nanocrystal Assemblies Determines Charge Transport of Films
Self-assembly of semiconductor nanocrystals
(NCs) into two-dimensional
patterns or three-dimensional (2-3D) superstructures has emerged as
a promising low-cost route to generate thin-film transistors and solar
cells with superior charge transport because of enhanced electronic
coupling between the NCs. Here, we show that lead sulfide (PbS) NCs
solids featuring either short-range (disordered glassy solids, GSs)
or long-range (superlattices, SLs) packing order are obtained solely
by controlling deposition conditions of colloidal solution of NCs.
In this study, we demonstrate the use of the evaporation-driven self-assembly
method results in PbS NC SL structures that are observed over an area
of 1 mm × 100 μm, with long-range translational order of
up to 100 nm. A number of ordered domains appear to have nucleated
simultaneously and grown together over the whole area, imparting a
polycrystalline texture to the 3D SL films. By contrast, a conventional,
optimized spin-coating deposition method results in PbS NC glassy
films with no translational symmetry and much shorter-range packing
order in agreement with state-of-the-art reports. Further, we investigate
the electronic properties of both SL and GS films, using a field-effect
transistor configuration as a test platform. The long-range ordering
of the PbS NCs into SLs leads to semiconducting NC-based solids, the
mobility (μ) of which is 3 orders of magnitude higher than that
of the disordered GSs. Moreover, although spin-cast GSs of PbS NCs
have weak ambipolar behavior with limited gate tunability, SLs of
PbS NCs show a clear p-type behavior with significantly higher conductivities
Images of silicon devices.
<p>a: Schematic drawing of the different regions on a silicon chip. b: Photo of the device together with a reference size. c: Scanning Electron Microscopy image of the three-dimensional silicon microstructure.</p
Comparison between fluorescence images relative to HT1080 cells in Photonic Crystals.
<p>a: PhC with long walls. b: PhC with short walls. No significant difference is observed between (a) and (b) since the length of the walls does not affect the cell behavior.</p
A New Cell-Selective Three-Dimensional Microincubator Based on Silicon Photonic Crystals
<div><p>In this work, we show that vertical, high aspect-ratio (HAR) photonic crystals (PhCs), consisting of periodic arrays of 5 µm wide gaps with depth of 50 µm separated by 3 µm thick silicon walls, fabricated by electrochemical micromachining, can be used as three-dimensional microincubators, allowing cell lines to be selectively grown into the gaps. Silicon micromachined dice incorporating regions with different surface profiles, namely flat silicon and deeply etched PhC, were used as microincubators for culturing adherent cell lines with different morphology and adhesion properties. We extensively investigated and compared the proliferative behavior on HAR PhCs of eight human cell models, with different origins, such as the epithelial (SW613-B3; HeLa; SW480; HCT116; HT29) and the mesenchymal (MRC-5V1; CF; HT1080). We also verified the contribution of cell sedimentation into the silicon gaps. Fluorescence microscopy analysis highlights that only cell lines that exhibit, in the tested culture condition, the behavior typical of the mesenchymal phenotype are able to penetrate into the gaps of the PhC, extending their body deeply in the narrow gaps between adjacent silicon walls, and to grow adherent to the vertical surfaces of silicon. Results reported in this work, confirmed in various experiments, strongly support our statement that such three-dimensional microstructures have selection capabilities with regard to the cell lines that can actively populate the narrow gaps. Cells with a mesenchymal phenotype could be exploited in the next future as bioreceptors, in combination with HAR PhC optical transducers, e.g., for label-free optical detection of cellular activities involving changes in cell adhesion and/or morphology (e.g., apoptosis) in a three-dimensional microenvironment.</p> </div
Fluorescence images relative to HT29 cells.
<p>a–b: Cell morphology on a glass slide (glass). c–d: Cell morphology in a flat silicon region (S). e–f: Cell morphology in the Photonic Crystal (PhC). Cells are labeled with (a, c, e) green-FITC and red-PI; (b, d, f) only red-PI.</p
Comparison between fluorescence images relative to HT1080 cell culture in different conditions on PhC with short walls.
<p>a: Standard culture. b: Cell deposition at room temperature (∼20°C). c: Cell deposition at 37°C.</p