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The control of acidity drives the assembly of biopolymers that are essential for a wide range of applications. Its miniaturization can increase the speed and the possibilities of combinatorial throughput for their manipulation, similar to the way that the miniaturization of transistors allows logical operations in microelectronics with a high throughput. Here, we present a device containing multiplexed microreactors, each one enabling independent electrochemical control of acidity in ∼2.5 nL volumes, with a large acidity range from pH 3 to 7 and an accuracy of at least 0.4 pH units. The attained pH within each microreactor (with footprints of ∼0.3 mm2 for each spot) was kept constant for long retention times (∼10 min) and over repeated cycles of >100. The acidity is driven by redox proton exchange reactions, which can be driven at different rates influencing the efficiency of the device in order to achieve more charge exchange (larger acidity range) or better reversibility. The achieved performance in acidity control, miniaturization, and the possibility to multiplex paves the way for the control of combinatorial chemistry through pH- and acidity controlled reactions.

Field Effect Transistors (FETs) have led the progress of applications measuring the acidity in aqueous solutions thanks to their accuracy, ease of miniaturization and capacity of multiplexing. The signal-to-noise ratio and linearity of the sensors are two of the most relevant figures of merit that can facilitate the improvements of these devices. In this work we present the functionalization with aminopropyltriethoxysilane (APTES) of a promising new FET design consisting of a high height-towidth aspect ratio with an efficient 2D gating architecture that improves both factors. We measured the transistor transfer and output characteristics before and after APTES functionalization, obtaining an improved sensitivity and linearity in both responses. We also compared the experimental results with a site-biding model of the surface buffering capacity of the APTES functionalized layers.

Protein sequencing is an important key to personalized medicines, but the process is complex enough to halt the ongoing progress in proteomics. In this paper, we have proposed a novel simulation methodology for the detection of oligopeptide fingerprints using a Field-Effect-Transistor. In our approach, the Gouy-Chapman-Stern and Site-Binding models are solved self-consistently to capture the response of immobilized peptides in the presence of an electrolyte. Our results show the unique signatures of two anti-hypertensive tripeptides in terms of variation in surface potential, inflection points and point-of-zero-charge in 2nd order differentiation of surface potential and total surface capacitance. We have shown that the presence of silanol sites, in presence of single or multiple oligopeptides, is responsible for reduced sensor’s sensitivity. We have proposed a novel noise-reduction technique to eliminate the noise present in the experimental data.

Protein sequencing is key to many biological fields to advance science and prospect medical applications based on proteomics. However, even with advanced techniques such as mass spectrometry it is hard to detect the fluctuations in AA sequences and optical/geometrical isomers, and cannot provide wide access to ex-novo sequencing of low quantities of proteins. We are presenting a new and disruptive method based on FET sensors for AA/polypeptide detection. We show unique signals (fingerprints) of every single AA and polypeptide mutations by solving the site-binding model self-consistently with the Gouy-Chapman-Stern model. The surface potential (Ψo), 2nd gradient of the surface potential (δΨo2/δpH2) and total surface capacitance (CT) are used as fingerprint signals to differentiate between the amino acids including the drain current variation as the signal transduction. These fingerprints are based on orthogonal properties of the AAs, which are the different proton affinities of each of the radicals, their dielectric constant and effective length, which is sensitive to the relative positions of the radicals to distinguish between isomers. We studied the generation of signals and proposed a novel noise-filtering technique based on the Fast-Fourier Transform (FFT) and the significantly improved analytical model which is solved iteratively to reduce data loss. The minimum combined fingerprint resolution is 0.1 units of pH for the separation of singularities found in δΨo2/δpH2, linked to the minimum capacitance of the AAs with a needed resolution of 0.01mF/m2 for surface densities of 1014cm-2, and which can be normalized to lower densities. The effect of noise (>SNR=10dB) and silanol sites can be negated by correlating the AAs signatures from δΨo2/δpH2 and capacitance. Thus, the designed methodology and approach can help immensely in designing a new and efficient tool for protein sequencing while solving the problems related to the signal transduction of sensors.

Controlling locally produced acidity in miniaturized spaces is of high importance yielding to manage simultaneous chemical reactions. Here we present a platform that hosts miniaturized micro reactors, each one enabling electrochemical control of the acidity in ~nL volumes. We demonstrated the local control of chemical reactions with the deprotection of strong acid labile groups in a region of 150 μm of diameter of an upstanding glass using high proton concentrations (~10-1M) and the acidity contrasts between the cell region and the outside. We demonstrated an accurate control of the proton concentration in aqueous and organic solvents and the control of chemical reactions in organic electrolytes achieved with a sulfonated tetrafluoroethylene-based membrane, that isolates the acid generating electrodes from the reagents in the solution. The quantitative control of the acidity by the Faradaic currents was demonstrated by the calibration of carboxyfluorescein adjusted with external titrations, and with a tautomer transition occurring at pH 4.2. To the best of our knowledge, this platform shows the best control of acidity in the smallest volume reported so far.

In this work, we show the control of acidity and chemical reactions in nanofluidic electrochemical reactors with water and organic solvents. We demonstrate the accurate control of the proton concentration using Faradaic cur-rents calibrated against carboxyfluorescein adjusted with external titrations, but also with a tautomer transition occurring at pH 4.2. We deployed our platform for the control of acidity with organic solvents using a modification of the electrodes with a sulfonated tetrafluoroethylene-based membrane, that isolates the acid generating electrodes from the reagents in the solution. This configuration al-lowed us to follow the acidity in the cell using the same carboxyfluorescein, observing no deterioration of the acid/base cycles, proving the effective isolation of the electrodes. Finally, due to the high proton concentration and the acidity contrast between the cell and the outside that can be achieved with our system, we performed the deprotection of acid labile groups in an upstanding glass in a region with 150 μm of diameter. To the best of our knowledge, this plat-form shows the best control of acidity in the smallest volume reported so far.

The development of next generation medicines demand more sensitive and reliable label free sensing able to cope with increasing needs of multiplexing and shorter times to results. Field effect transistor-based biosensors emerge as one of the main possible technologies to cover the existing gap. The general trend for the sensors has been miniaturisation with the expectation of improving sensitivity and response time, but presenting issues with reproducibility and noise level. Here we propose a Fin-Field Effect Transistor (FinFET) with a high heigth to width aspect ratio for electrochemical biosensing solving the issue of nanosensors in terms of reproducibility and noise, while keeping the fast response time. We fabricated different devices and characterised their performance with their response to the pH changes that fitted to a Nernst-Poisson model. The experimental data were compared with simulations of devices with different aspect ratio, stablishing an advantage in total signal and linearity for the FinFETs with higher aspect ratio. In addition, these FinFETs promise the optimisation of reliability and efficiency in terms of limits of detection, for which the interplay of the size and geometry of the sensor with the diffusion of the analytes plays a pivotal role., Comment: Article submitted to Nano Letters

The electrochemical management of the proton concentration in miniaturized dimensions opens the way to control and parallelize multistep chemical reactions, but still it faces many challenges linked to the efficient proton generation and control of their diffusion. Here we present a device operated electrochemically that demonstrates the control of the pH in a cell of ∼140 nL. The device comprises a microfluidic reactor integrated with a pneumatic mechanism that allows the exchange of reagents and the isolation of protons to decrease the effect of their diffusion. We monitored the pH with a fluorescence marker and calculated the final value from the redox currents. We demonstrate a large pH amplitude control from neutral pH values beyond the fluorescence marker range at pH 5. On the basis of the calculations from the Faradaic currents, the minimum pH reached should undergo pH ∼ 0.9. The pH contrast between neutral and acid pH cells can be maintained during periods longer than 15 min with an appropriate design of a diffusion barrier.

In this work we combine a Fin Field Effect Transistor (Fin-FET) characterised by a high height to width aspect ratio with high-k dielectric materials to study the optimized design for chemical-FETs to provide higher transconductance (and thus a better signal to noise ratio), increased dynamic range and chemical stability. We used pH sensing to verify the design. We explored the sensitivity and response linearity of silicon dioxide, alumina and hafnium oxide as dielectric materials sensing pH, and compared their chemical stability in different acids. The high aspect ratio fin geometry of the sensor provides high currents, as well as a planar conduction channel more reliable than traditional silicon nanowires. The hafnium oxide Fin-FET configuration performed the best delivering the most linear response both for the output and transfer characteristics providing a wider dynamic range. Hafnium oxide also showed the best chemical stability. Thus, we believe that the developed high aspect ratio Fin-FETs/high-k dielectric system can offer the best compromise of performance of FET-based sensors., 17 pages, 4 figures

© 2020 The Author(s)., [Introduction]: The importance of biomarkers for pharmaceutical drug development and clinical diagnostics is more significant than ever in the current shift toward personalized medicine. Biomarkers have taken a central position either as companion markers to support drug development and patient selection, or as indicators aiming to detect the earliest perturbations indicative of disease, minimizing therapeutic intervention or even enabling disease reversal. Protein biomarkers are of particular interest given their central role in biochemical pathways. Hence, capabilities to analyze multiple protein biomarkers in one assay are highly interesting for biomedical research. [Areas covered]: We here review multiple methods that are suitable for robust, high throughput, standardized, and affordable analysis of protein biomarkers in a multiplex format. We describe innovative developments in immunoassays, the vanguard of methods in clinical laboratories, and mass spectrometry, increasingly implemented for protein biomarker analysis. Moreover, emerging techniques are discussed with potentially improved protein capture, separation, and detection that will further boost multiplex analyses. [Expert commentary]: The development of clinically applied multiplex protein biomarker assays is essential as multi-protein signatures provide more comprehensive information about biological systems than single biomarkers, leading to improved insights in mechanisms of disease, diagnostics, and the effect of personalized medicine., This paper was funded by the European cooperation in science and technology - COST action No. CA16113 - CliniMARK: ‘good biomarker practice’ to increase the number of clinically validated biomarkers.

We present a review of field effect transistors (FET) from the point of view of their applications to label-free sensing in the era of genomics and proteomics. Here, rather than a collection of Bio-FET achievements, we propose an analysis of the different issues hampering the use of these devices into clinical applications. We make a particular emphasis on the influence of the sensor geometry in the phenomena of mass transport of analytes, which is a topic that has been traditionally overlooked in the analysis and design of biosensors, but that plays a central role in the achievement of low limits of detection. Other issues like the screening of charges by the ions in liquids with physiological ionic strength and the non-specific binding are also reviewed. In conclusion, we give an overview of different solutions that have been proposed to address all these challenges, demonstrating the potential of field effect transistors owing to their ease of integration with other semiconductor components for developing cost-effective, highly multiplexed sensors for next-generation medicines.

Temperature sensing is one of the important features of Micro Electro Mechanical Systems and a monolithic integration provides advantages for both fabrication simplicity and performance. The use of Silicon On Insulator substrates allows simple fabrication of integrated wires that can be used as thermistors. We fabricated rectangular and triangular silicon wires with different dimensions in a single step fabrication process based on the wet etching of a Silicon On Insulator substrate. We determined the experimental resistivity of the two kinds of devices and tested their performance as thermistors in a temperature range between 24 and 100 °C. The accuracy and normalized sensitivities of our devices were 0.4 °C and 0.3–0.5%/°C, respectively. The potential of the proposed method resides in the possibility of having devices with different shapes in a single straightforward process.

We made an error in the designation of the voltage offset used for the calculation of the shift of the surface potential. The method that we used with the Vref having a minimum in δIds/δVref calculates the threshold voltage and not flat band potential. As both offsets are equivalent to follow the changes in ψ0, so there is no significant change in the interpretation of the data. The labels and legend for Figure 2 in the original article should be corrected as shown here. (Figure presented).

Using electrochemical control to change the chemical environment in liquids is quite challenging because the small surface to volume ratio offered by traditional cells limits the release or modification of chemicals from the electrodes. Here we present the control of proton concentration (acidity) using a combination of a redox active SAM, which allows ease of fabrication, control of the chemical reactions and repeatability, with a microfluidic design that provides large range and stability over time of the set acidity point, quasi-reversibility during several cycles, and capacity of miniaturization and scalability for multiplexing.

The electrochemical control of molecular concentrations in liquids is quite challenging because of the small ratio of interacting sites between the electrode surface and the number of molecules in the volume. Here we review our recent works aiming the control of proton concentration combining redox self-assembled monolayers with a modified platform. We used Aminothiolphenol, to obtain coatings with quasi-reversible redox states able to exchange protons with the electrolyte at low voltages using different polymerization methods. We studied their charge exchange during different cycles, which would provide us the possibility to use different cycles of reactions controlled by acidity. We achieved the control of proton concentration over the liquid with an efficient design of the microfluidic device to control the diffusion of protons avoiding the reduction of hydrogen at the counter electrode and providing long lasting stability to carry control of chemical reactions during several tenths of minutes. The experiments were carried out in aqueous environment using a pH fluorescence marker to track the pH that allowed us to monitor the proton concentration down to pH 5, where the fluorescence molecule lost its sensitivity, while calculations indicate that the pH can be below 1.

The reversibility of redox processes is an important function for sensing and molecular electronic devices such as pH reporters or molecular switches. Here we report the electrochemical behaviour and redox reversibility of para-aminothiolphenol (PATP) after different polymerisation methods. We used electrochemical and photo-polymerisation in neutral buffers and plasma polymerisation in air to induce reversible redox states. The chemical stoichiometry and surface coverage of PATP in the polymerized layers were characterized by X-ray photoelectron spectroscopy (XPS), while cyclic voltammetry (CV) was used to measure the charge transfer, double layer capacitance and electrochemical rate of the layers during successive potential cycles. Our results show that the surface coverage of the redox active species is higher on electro-polymerised samples, however, after consecutive cycles all the methods converge to the same charge transfer, while the plasma polymerised samples achieve higher efficiency per molecule and UV polymerised samples have a higher electron transfer rate.

Interleukin 1 beta (IL-1β)-dependent inflammatory disorders, such as rheumatoid arthritis and psoriasis, pose a serious medical burden worldwide, where patients face a lifetime of illness and treatment. Organogold compounds have been used since the 1930s to treat rheumatic and other IL-1β-dependent diseases and, though their mechanisms of action are still unclear, there is evidence that gold interferes with the transmission of inflammatory signalling. Here we show for the first time that citrate-stabilized gold nanoparticles, in a size dependent manner, specifically downregulate cellular responses induced by IL-1β both in vitro and in vivo. Our results indicate that the anti-inflammatory activity of gold nanoparticles is associated with an extracellular interaction with IL-1β, thus opening potentially novel options for further therapeutic applications.

The aim of the present study was to investigate the effect of silver nanoparticles (AgNP) of different sizes toward two primary producer aquatic species. Thalassiosira pseudonana and Synechococcus sp. have been selected as representative models for the lower trophic organisms in marine and freshwater habitats, respectively. Time-dependent cellular growth was measured upon exposure to both AgNP and silver nitrate (AgNO(3)). In addition, AgNP behavior in freshwater and marine waters has been followed by CPS disc centrifuge, in the time frame of AgNP exposure studies, and the kinetic release of silver from AgNP of different sizes was measured by dialysis and inductively coupled plasma mass spectrometry (ICP-MS). The combination and interpretation of all these data suggest that a shared effect of AgNP and released silver was responsible for the toxicity in both organisms. Furthermore, the toxic effects induced by AgNP exposure in the present study seem to result from a mixture of parameters including aggregated state, size of the AgNP, stability of the preparation, and speciation of the released silver.

We present a novel approach to tackle the most common drawback of using surface plasmon resonance for analyte screening in complex biological matrices - the nonspecific binding to the sensor chip surface. By using a perforated membrane supported by a polymeric gel structure at the evanescent wave penetration depth, we have fabricated a non-fouling sieve above the sensing region. The sieve shields the evanescent wave from nonspecific interactions which interfere with SPR sensing by minimizing the fouled area of the polymeric gel and preventing the translocation of large particles, e. g. micelles or aggregates. The nanopatterned macropores were fabricated by means of colloidal lithography and plasma enhanced chemical vapor deposition of a polyethylene oxide-like film on top of a polymeric gel matrix commonly used in surface plasmon resonance analysis. The sieve was characterized using surface plasmon resonance imaging, contact angle, atomic force microscopy and scanning electron microscopy. The performance of the sieve was studied using an immunoassay for detection of antibiotic residues in full fat milk and porcine serum. The non-fouling membrane presented pores in the 92-138 nm range organized in a hexagonal crystal lattice with a clearance of about 5% of the total surface. Functionally, the membrane with the nanopatterned macropores showed significant improvements in immunoassay robustness and sensitivity in untreated complex samples. The utilization of the sensor built-in sieve for measurements in complex matrices offers reduction in pre-analytical sample preparation steps and thus shortens the total analysis time.

Low-lying collective excitations above highly correlated ground states of few interacting electrons confined in GaAs semiconductor quantum dots are probed by resonant inelastic light scattering. We highlight that separate studies of the changes in the spin and charge degrees of freedom offer unique access to the fundamental interactions. The case of quantum dots with four electrons is found to be determined by a competition between triplet and singlet ground states that is uncovered in the rich light scattering spectra of spin excitations. These light scattering results are described within a configuration-interaction framework that captures the role of electron correlation with quantitative accuracy. Recent light scattering results that reveal the impact of anisotropic confining potentials in laterally coupled quantum dots are also reviewed. In these studies, inelastic light scattering methods emerge as powerful probes of collective phenomena and spin configurations in quantum dots with few electrons.

We present chemical modification of self assembled monolayers (SAMs) using electron and ion-beam lithographies. We used thiolated polyethylene oxide (PEO) SAMs on gold to fabricate chemically contrasting patterns at the nanoscale. Patterned surfaces were characterized by X-ray photoelectron spectroscopy (XPS), time of flight-secondary ion mass spectrometry (ToF-SIMS). Results showed a chemical modification of surfaces patterned by means of electron beam (e-beam) lithography and a removal of PEO SAMs on the areas treated with the ion beam. The chemical modification of PEO SAMs converted the non-fouling surfaces on fouling surfaces.

Spin transitions and correlated few-electron states are investigated by resonant inelastic light scattering in dilute arrays of GaAs/AlGaAs modulation-doped quantum dots (QDs) fabricated by electron-beam lithography and low impact reactive-ion etching. We focus on QDs with four electrons. We show that at moderate magnetic fields, the ground state is a singlet with total spin S = 0. A rich spectrum of distinct spin and charge inter-shell excitations is found, which cannot be described by a mean-field Hartree-Fock framework based on the quantum description of Fock-Darwin energy levels. Instead, the experimental results are well modeled by numerical evaluations within a full configuration interaction approach that highlights the impact of correlation effects in this configuration. This work demonstrates that the sensitivity reached by resonant inelastic light scattering enables the study of few-electron effects in QDs formed by state-of-the-art nanofabrication processes under the extreme conditions of low temperatures and high magnetic fields. (C) 2007 Elsevier B.V. All rights reserved.

In the following article an electron/ion microscopy study will be presented which investigates the uptake of silver nanoparticles (AgNPs) by the marine diatom Thalassiosira pseudonana, a primary producer aquatic species. This organism has a characteristic silica exoskeleton that may represent a barrier for the uptake of some chemical pollutants, including nanoparticles (NPs), but that presents a technical challenge when attempting to use electron-microscopy (EM) methods to study NP uptake. Here we present a convenient method to detect the NPs interacting with the diatom cell. It is based on a fixation procedure involving critical point drying which, without prior slicing of the cell, allows its inspection using transmission electron microscopy. Employing a combination of electron and ion microscopy techniques to selectively cut the cell where the NPs were detected, we are able to demonstrate and visualize for the first time the presence of AgNPs inside the cell membrane.

We report here in vitro evaluation of cellular nanoparticle uptake, cytotoxic effect and autophagy as possible survival strategy in lung epithelial cells (A549). The effects of surface charge and presence of serum proteins, as well as the influence of various inhibitors on the uptake of 20 nm monodispersed silica nanoparticles are discussed. Uptake rate of the particles with various functional groups is demonstrated to be similar in the presence of serum proteins, while the uptake rate ranking results in COOH > NH2 > OH under serum free conditions. The uptake process is shown to be actin dependent, and can be partially inhibited by dynasore in the presence of serum proteins. These results suggest a macropinocytotic uptake process that was also confirmed by scanning electron microscopy. In spite of the intensive active uptake, significant cytotoxic effect is detected only at relatively high (above 250 μg/mL) concentrations, while blebbing of the cell surface is observed at already 5 hours of exposure time. By monodansylacadaverin staining and RT-PCR experiments, we show that this blebbing is related to autophagy rather than apoptotic cell death. After the exposure to silica nanoparticles, stress-related and cell-death-related genes are found to be upregulated, but no activation of the apoptotic process was observed. The A549 cells display elevated levels of autophagosomes, however cells do not show typical apoptosis markers such as increased amount of active caspase-3 and release of mitochondrial cytochrome C. Based on these results, we propose here autophagic activity and cross-talk between autophagic and apoptotic pathways as a mechanism that allows the survival of A549 cells under exposure to silica nanoparticles., JRC.I.4-Nanobiosciences

Gold nanoparticles (AuNPs) can interfere with some of the biochemical processes of macrophage cells but the mechanisms of action of these potentially medically-relevant effects are still unclear. Here we study the fate of AuNPs interacting with cells derived from the innate immune system. To visualize AuNPs with nanometer resolution without losing sight of the whole morphology of cells we developed a convenient approach that uses electron and ion microscopy techniques. The inspection using an ion beam to selectively cut where the AuNPs were found, allows for determining their intracellular localizations. We studied the cellular uptake of AuNPs, with or without exposure of the cells to Latrunculin-A, a phagocytosis inhibitor. Results indicate a size dependence of the internalization mechanisms for THP-1 cells. The internalization of larger AuNPs was blocked in the presence of Latrunculin-A, although they could attach to the membrane. Smaller AuNPs though were not blocked by actin depending processes.

Surface Enhanced Raman Spectroscopy (SERS) is a popular method in bio-analytical chemistry and a potentially powerful enabling technology for in vitro diagnostics. SERS combines the excellent chemical specificity of Raman spectroscopy with the good sensitivity provided by enhancement of the signal that is observed when a molecule is located on (or very close to) the surface of nanostructured metallic materials. Star-like gold nanoparticles (SGN) are a new class of multibranched nanoparticles that in the last few years have attracted the attention of SERS community for their plasmonic properties. In this work we present a new method to prepare star-like gold nanoparticles with a simple one step protocol at room temperature using hydroquinone as reducing agent. Besides we compare the enhancement of Raman signal of malachite green, a dye commonly employed as label in biological studies, by star-like gold nanoparticles having different size, directly in liquid. This study shows that SGN provide good enhancement of Raman signal and that the effect of their dimension is strongly dependent on the wavelength used. Moreover preliminary results suggest that SGN produced using this method are characterized by good physical-chemical properties and they can be functionalized using the standard thiol chemistry. Overall, these results suggest that star-like gold nanoparticles produced through this method could be used for the further development of highly specific and sensitive SERS-based bio-analytical tests. © 2013 Copyright SPIE.

Gold nanoparticles (AuNPs) are currently used in several fields including biomedical applications, although no conclusive information on their cytotoxicity is available. For this reason this work has investigated the effects of AuNPs in vitro on Balb/3T3 mouse fibroblasts. Results obtained exposing cells for 72 h to AuNPs 5 and 15 nm citrate stabilized, revealed cytotoxic effects only for AuNPs 5 nm at concentration ≥ 50 μM if measured by colony forming efficiency (CFE). To understand the differences in cytotoxicity observed for the two AuNPs sizes, we investigated the uptake and the intracellular distribution of the nanoparticles. By TEM it was observed that 5 and 15 nm AuNPs are internalized by Balb/3T3 cells and located within intracellular endosomal compartments. Quantification of the uptake by ICP-MS showed that AuNPs internalization enhanced even up to 72 h. Disruption of the actin cytoskeleton was evident, with cell footprints narrow and contracted; effects more remarkable in cells exposed to 5 nm AuNP. The mechanism of NPs cell internalization was investigated using immunocytochemistry and western blot. No significant effect was observed in the expression level of caveolin, while reduction of the expression and degradation of the clathrin heavy chain was observed in cells exposed for 72 h to AuNPs.

We show that using asymmetric flow-field-flow fractionation and UV-vis detector it is possible to separate, characterize, and quantify the correct number size distribution of gold nanoparticle (AuNP) mixtures of various sizes in the 5-60 nm range for which simple dynamic light scattering measurements give misleading information. The size of the collected nanoparticles fractions can be determined both in solution and in the solid state, and their surface chemistry characterized by NMR. This method will find widespread applications both in the process of "size purification" after the synthesis of AuNP and in the identification and characterization of gold-based nanomaterials in consumer products., JRC.DG.I.5-Nanobiosciences

We observed room-temperature intersubband transitions at 1.26 microns in n-doped type-II In$_{0.3}$Ga$_{0.7}$As/AlAs strained quantum wells. An improved tight-binding model was used to optimize the structure parameters in order to obtain the shortest wavelength intersubband transition ever achieved in a semiconductor system. The corresponding transitions occur between the first confined electronic levels of the well following mid-infrared optical pumping of electrons from the barrier X-valley into the well ground state., Comment: To appear in Applied Physics Letters

Correlation among particles in finite quantum systems leads to complex behaviour and novel states of matter. One remarkable example is predicted to occur in a semiconductor quantum dot (QD) where at vanishing density the Coulomb correlation among electrons rigidly fixes their relative position as that of the nuclei in a molecule. In this limit, the neutral few-body excitations are roto-vibrations, which have either rigid-rotor or relative-motion character. In the weak-correlation regime, on the contrary, the Coriolis force mixes rotational and vibrational motions. Here we report evidence of roto-vibrational modes of an electron molecular state at densities for which electron localization is not yet fully achieved. We probe these collective modes by inelastic light scattering in QDs containing four electrons. Spectra of low-lying excitations associated to changes of the relative-motion wave function -the analogues of the vibration modes of a conventional molecule- do not depend on the rotational state represented by the total angular momentum. Theoretical simulations via the configuration-interaction (CI) method are in agreement with the observed roto-vibrational modes and indicate that such molecular excitations develop at the onset of short-range correlation., PDF file only; 24 pages, 7 figures, 2 table. Supplementary Information included

We present recent studies of electronic excitations in nanofabricated AlGaAs/GaAs semiconductor quantum dots (QDs) by resonant inelastic light scattering. The resonant light scattering spectra are dominated by excitations from parity-allowed inter-shell transitions between Fock-Darwin levels. In QDs With very few electrons the resonant spectra are characterized by distinct charge and spin excitations that reveal the strong impact of both exchange and correlation effects. A sharp inter-shell spin excitation of the triplet spin QD state with four electrons is identified. (c) 2006 Elsevier B.V. All rights reserved.

Tunneling excitations of electrons in dry-etched modulation-doped AlGaAs/GaAs coupled quantum dots (QDs) are probed by resonant inelastic light scattering. A sequence of intra- and intershell excitations are found at energies determined by the interplay between the QD confinement energy $\hbar \omega_0$ and the tunneling gap $\Delta_{SAS}$, the splitting between the symmetric and anti-symmetric delocalized single particle molecular states. The narrow line-widths displayed by electronic excitations in these nanostructures indicate promising venues for the spectroscopic investigation of entanglement of electron states in these artificial molecules., Comment: 8 pages, single column, including 3 figures

We report inelastic light scattering measurements of spin and charge excitations in nanofabricated AlGaAs/GaAs quantum dots with few electrons. A narrow spin excitation peak is observed and assigned to the intershell triplet-to-singlet monopole mode of dots with four electrons. Configurationinteraction theory provides precise quantitative interpretations that uncover large correlation effects that are comparable to exchange Coulomb interactions., Comment: 4 pages, 4 figures