Miniaturized solid state capacitors leveraging migration of unipolar ions in a single polyelectrolyte layer sandwiched between metal electrodes, namely, polyelectrolyte capacitors (PECs), have been recently reported with areal capacitance up to 100-200 nF mm-2. Nonetheless, application of PECs in consumer and industrial electronics has been hindered so far by their small operational frequency range, up to a few kHz, due to the resistive behavior (phase angle >-45°) of PECs in the range kHz-to-MHz. Here, it is reported on multilayer polyelectrolyte capacitors (mPECs) that leverage as dielectric an ambipolar nanometer-thick (down to 10 nm) stack of anionic and cationic polyelectrolytes assembled layer-by-layer between metal electrodes to eliminate the resistive behavior at frequencies from kHz to MHz. This significantly extends the operational range of mPECs over PECs. mPECs with areal capacitance as high as 25 nF mm-2 at 20 Hz and full capacitive behavior from 100 mHz to 10 MHz are demonstrated using different assembling conditions and anionic/cationic polyelectrolyte pairs. The mPECs reliably operate over time for >300 million cycles, at different biasing voltages up to 3 V, and temperatures up to 80 °C, showing a reversible capacitive behavior without significant hysteresis. Application of mPECs in flexible electronics, also operating at high frequency, is envisaged.

A new instrument for label-free measurements based on optical Low Q-Whispering Gallery Modes for various applications is presented. Fluorescent spherical beads with diameters of 7-12 mu m serve as sensors, detecting analyte quantities in femtogram range and can be used independently of the measuring device in smallest spaces. Spherical beads with a very smooth surface and high refractive index serve as resonators for circular light waves. For the measurement, the fluorescent bead is excited with a laser. Part of the fluorescence is totally reflected back into the particle, so that circulating waves are formed. They circulate up to 10,000 times in the particle. Their resonance frequency depends sensitively on particle diameter and refractive index difference between particle and medium. When molecules are adsorbed on the surface, the resonant frequency shifts because both parameters change. A robust and compact instrument with excitation laser, high resolution spectrometer, CCD array, xyz-stage, microscope, camera, instrument control and evaluation software has been developed to measure this shift. Some application examples in biosensing, layer-by-layer deposition of polymers, drug release and solvent sensing are presented. They show the broad potential of this new measurement methodology due to clear advantages over alternative methods on the market.

Zusammenfassung Es wird ein neues Instrument für labelfreie Messungen auf der Basis von optischen Low Q-Whispering Gallery Modes vorgestellt, bei der mit miniaturisierten Sensorbeads verschiedene Anwendungsfelder adressiert werden. Als Sensoren dienen fluoreszente sphärische Beads mit Durchmessern von 7–12 Mikrometern, die unabhängig vom Messgerät in kleinsten Räumen eingesetzt werden können und mit Analytmengen im Femtogrammbereich auskommen. Die sphärischen Beads mit einer sehr glatten Oberfläche und hohem Brechungsindex dienen als Resonatoren für zirkulare Lichtwellen. Für die Messung wird der Fluoreszenzfarbstoff in den Sensoren mit einem Laser angeregt. Ein Teil der Fluoreszenz wird an der Partikeloberfläche total ins Partikel zurück reflektiert, so dass sich zirkulierende Wellen bilden, die bis zu 10,000 Mal umlaufen. Ihre Resonanzfrequenz hängt empfindlich vom Partikeldurchmesser und der Brechungsindexdifferenz zwischen Partikel und Medium ab. Bei Adsorbtion von Molekülen verschiebt sich die Resonanzfrequenz, da sich beide Parameter ändern. Für die Messung dieser Verschiebung wurde ein kompaktes Gerät mit Anregungslaser, hochauflösendem Spektrometer, CCD-Zeile, XYZ -Stage, Mikroskop, Kamera, Gerätesteuerung und Auswertesoftware entwickelt. Es werden einige Anwendungsbeispiele in der Biosensorik, der Layer-by-Layer Beschichtung, der Wirkstofffreisetzung und der Lösungsmittelsensorik präsentiert, die das breite Potential und Vorteile gegenüber auf dem Markt befindlicher, alternativer Meßmethoden demonstrieren.

Here, the authors report on the manufacturing and in vivo assessment of a bioresorbable nanostructured pH sensor. The sensor consists of a micrometer-thick porous silica membrane conformably coated layer-by-layer with a nanometer-thick multilayer stack of two polyelectrolytes labeled with a pH-insensitive fluorophore. The sensor fluorescence changes linearly with the pH value in the range 4 to 7.5 upon swelling/shrinking of the polymer multilayer and enables performing real-time measurements of the pH level with high stability, reproducibility, and accuracy, over 100 h of continuous operation. In vivo studies carried out implanting the sensor in the subcutis on the back of mice confirm real-time monitoring of the local pH level through skin. Full degradation of the pH sensor occurs in one week from implant in the animal model, and its biocompatibility after 2 months is confirmed by histological and fluorescence analyses. The proposed approach can be extended to the detection of other (bio)markers in vivo by engineering the functionality of one (at least) of the polyelectrolytes with suitable receptors, thus paving the way to implantable bioresorbable chemical sensors.

Here we report on a bioresorbable fluorescence sensor for in vivo pH monitoring. The sensor leverages a nanometer-thick multilayer stack of polyelectrolytes labelled with a pH-insensitive fluorophore conformably deposited within a porous silica membrane—thickness of a few micrometers—to increase fluorescence intensity up to 600 times and enable reliable measurements through skin.

Porous silicon (PSi) is a promising material for future integrated nanophotonics when coupled with guest emitters, still facing challenges in terms of homogenous distribution and nanometric thickness of the emitter coating within the silicon nanostructure. Herein, it is shown that the nanopore surface of a porous silicon oxide (PSiO 2 ) microcavity (MC) can be conformally coated with a uniform nm‐thick layer of a cationic light‐emitting polyelectrolyte, e.g., poly(allylamine hydrochloride) labeled with Rhodamine B (PAH‐RhoB), leveraging the self‐tuned electrostatic interaction of the positively‐charged PAH‐RhoB polymer and negatively‐charged PSiO 2 surface. It is found that the emission of PAH‐RhoB in the PSiO 2 MC is enhanced ( ≈ 2.5 × ) and narrowed ( ≈ 30 × ) at the resonant wavelength, compared with that of PAH‐RhoB in a non‐resonant PSiO 2 reference structure. The time‐resolved photoluminescence analysis highlights a shortening ( ≈ 20%) of the PAH‐RhoB emission lifetime in the PSiO 2 MC at the resonance versus off‐resonance wavelengths, and with respect to the reference structure, thereby proving a significant variation of the radiative decay rate. Remarkably, an experimental Purcell factor F p = 2.82 is achieved. This is further confirmed by the enhancement of the photoluminescence quantum yield of the PAH‐RhoB in the PSiO 2 MC with respect to the reference structure. Application of the electrostatic nanoassembling approach to other emitting dyes, nanomaterials, and nanophotonic systems is envisaged.

Here we report the layer-by-layer (LbL) nanoassembly of charged polyelectrolytes engineered with bioreceptors as an effective and robust alternative to covalent biofunctionalization (e.g., organosilanization and hydrosilylation) for affinity biosensing with porous silicon (PSi) interferometers [1]. As a proof of concept demonstration, a bi-layer of positively-charged poly(allylamine hydrochloride) (PAH) and negatively-charged biotinylated poly(methacrylic acid) (b-PMAA) is assembled onto the surface of oxidized PSi interferometers for the affinity detection of streptavidin in buffer and raw saliva. The LbL nanoassembly allows a homogenous coating of the inner PSi surface, ensuring a robust anchoring of the bioreceptors and, in turn, high sensitive and selective detection of streptavidin, also in raw saliva, down to a theoretical detection limit of 600 fM. This pushes PSi based biosensors at detection limit comparable to that of state-of-the-art nanostructured photonic and plasmonic platforms for biosensing. Further, the development and employment of polymers engineered with several bioreceptors or with stronger dissociation properties (e.g. PSS) could broaden the applications LbL nano-assembly in biosensing and in biomedical applications. [1] S. Mariani et al., Nature Communications. 2018, 9, 5256, 1-13.

Nanostructured materials hold the promise to revolutionize the label-free biosensing of analytes at concentrations down to those required for clinical applications, beyond what microstructured materials did so far, by leveraging the deeper interaction between materials and analytes with comparable size. On the other hand, when the characteristic dimension of the materials gets down to the nanoscale, the biofunctionalization chemistry commonly used to promote the binding of bioreceptors and enable, in turn, the specific detection of target analytes becomes less effective. Here, we propose an effective and robust route for the surface biofunctionalization of nanostructured materials based on the layer-by-layer electrostatic nano-assembling of oppositely-charged polyelectrolytes, which were engineered with bioreceptors covalently linked to the polymer chain. LbL biofunctionalization allows stability, density, and distribution of bioreceptors available on the transducer surface to be carefully controlled, thus solving most of the drawbacks of the common biofunctionalization chemistry on nanostructured materials. The proof-of-concept demonstration of advantages of LbL biofunctionalization is given on nanostuctured porous silicon (PSi) interferometers, which are biofunctionalized for the affinity detection of streptavidin in raw saliva via LbL nano-assembling of a positively-charged poly(allylamine hydrochloride) (PAH) with a negatively-charged biotinylated poly(methacrylic acid) (b-PMAA). LbL-biofunctionalizion of PSi interferometers results to be very effective and highly robust, enabling high-sensitivity and high-specificity detection of streptavidin with a detection limit of 600 fM. This represents a 100000-fold improvement with respect to control PSi interferometers biofunctionalized using common silane-based chemistry and a 300-fold improvement with respect to best PSi label-free biosensors reported in the current literature, pushing PSi optical biosensors to performance comparable to those of best label-free plasmonic and photonic platforms. [1] Layer-by-layer biofunctionalization of nanostructured porous silicon for high-sensitivity and high-selectivity label-free affinity biosensing, S Mariani, V Robbiano, LM Strambini, A Debrassi, G Egri, L Dähne, G. Barillaro, Nature Communications 9 (1), 5256 (2018)

Gold nanoparticle layers (AuNPLs) enable the coupling of morphological, optical, and electrical properties of gold nanoparticles (AuNPs) with tailored and specific surface topography, making them exploitable in many bioapplications (e.g., biosensing, drug delivery, and photothermal therapy). Herein, we report the formation of AuNPLs on porous silicon (PSi) interferometers and distributed Bragg reflectors (DBRs) for (bio)sensing applications via layer-by-layer (LbL) nanoassembling of a positively charged polyelectrolyte, namely, poly(allylamine hydrochloride) (PAH), and negatively charged citrate-capped AuNPs. Decoration of PSi interferometers with AuNPLs enhances the Fabry-Pérot fringe contrast due to increased surface reflectivity, resulting in an augmented sensitivity for both bulk and surface refractive index sensing, namely, about 4.5-fold using NaCl aqueous solutions to infiltrate the pores and 2.6-fold for unspecific bovine serum albumin (BSA) adsorption on the pore surface, respectively. Sensitivity enhancing, about 2.5-fold, is also confirmed for affinity and selective biosensing of streptavidin using a biotinylated polymer, namely, negatively charged poly(methacrylic acid) (b-PMAA). Further, decoration of PSi DBR with AuNPLs envisages building up a hybrid photonic/plasmonic optical sensing platform. Both photonic (DBR stop-band) and plasmonic (localized surface plasmon resonance, LSPR) peaks of the hybrid structure are sensitive to changes of bulk (using glucose aqueous solutions) and surface (due to BSA unspecific adsorption) refractive index. To the best of our knowledge, this is the first report about the formation of AuNPLs via LbL nanoassembly on PSi for (i) the enhancing of the interferometric performance in (bio)sensing applications and (ii) the building up of hybrid photonic/plasmonic platforms for sensing and perspective biosensing applications.

Low‐Q‐whispering gallery modes (low‐Q‐WGM) can be used for label‐free detection of interactions between biomolecules, measuring their binding and release kinetics or for analysis of changes in the medium in real‐time. The main advantage of the low‐Q‐WGM approach over other label‐free methods is the possibility of measurements in small cavities as the method uses microparticles down to 6 µm as sensors. Commercially available dye‐doped microparticles that are used as low‐Q‐WGM sensors exhibit several drawbacks. Therefore, alternative particle types are developed and optimized as low‐Q‐WGM sensors. First, dye‐doped particles made of different materials are screened. The most critical parameter for WGM performance is the refractive index (RI) of sensor particles. Furthermore, surface roughness of particles, determined by scanning electron microscopy and atomic force microscopy, affects their performance as WGM microsensors. In the second test, fluorescent dyes immobilized on nonfluorescent particles by means of nanometer thick layer‐by‐layer (LbL) films are shown to generate a strong WGM signal. The LbL‐coated particles show remarkably less background fluorescence than dye‐doped particles and are easier to prepare. Finally, this article proposes rapid preparation methods for WGM microparticle sensors based on various parameters such as material type, RI, surface roughness, and number of coated polymer layers.

The nano-assembly of charged polyelectrolytes via layer-by-layer (LbL) technology on porous silicon (PSi) interferometers is here demonstrated as an effective biofunctionalization approach for high-sensitivity/selectivity label-free optical biosensing, using streptavidin/biotin affinity detection as case study. Nanostructured PSi interferometers are biofunctionalized with a nano-assembly of a positively-charged polyelectrolyte, namely, PAH (poly(allylamine hydrochloride)), and a negatively-charged biotinylated polyelectrolyte, namely, b-PMAA (poly(methacrylic acid))), via LbL technology. The nano-assembly is stable under operating conditions and enables the selective and sensitive detection of streptavidin with a sub-picomolar detection limit (namely, DL=0.6 pM), which is 105-fold lower than that achieved with PSi interferometers biofunctionalized using standard silane chemistry. Remarkably, the analytical performance achieved for LbL-biofunctionalized PSi interferometers is comparable to those of state-of-the-art label-free photonic and plasmonic platforms.

Nanostructured materials premise to revolutionize the label-free biosensing of analytes for clinical applications, leveraging the deeper interaction between materials and analytes with comparable size. However, when the characteristic dimension of the materials reduces to the nanoscale, the surface functionalization for the binding of bioreceptors becomes a complex issue that can affect the performance of label-free biosensors. Here we report on an effective and robust route for surface biofunctionalization of nanostructured materials based on the layer-by-layer (LbL) electrostatic nano-assembly of oppositely-charged polyelectrolytes, which are engineered with bioreceptors to enable label-free detection of target analytes. LbL biofunctionalization is demonstrated using nanostructured porous silicon (PSi) interferometers for affinity detection of streptavidin in saliva, through LbL nano-assembly of a bi-layer of positively-charged poly(allylamine hydrochloride) (PAH) and negatively-charged biotinylated poly(methacrylic acid) (b-PMAA). High sensitivity in streptavidin detection is achieved, with high selectivity and stability, down to a detection limit of 600 fM.