Insights into Photopolymerization at the Nanoscale Using Surface Plasmon Resonance Imaging.

Near-field photopolymerization (NFPP) driven by surface plasmon resonance has attracted increasing attention in nanofabrication. This interest comes from the nanometer-scale control of polymer thickness, due to the confinement of the evanescent wave within a highly restricted volume at the surface. In this study, a novel approach using a multi-spectral surface plasmon resonance instrument is presented that gives access to real-time images of polymer growth during NFPP with nanometer sensitivity. Using the plasmonic evanescent wave for both polymerization and real-time sensing, the influence of irradiance, concentration of dye, and initiator are investigated on the threshold energy and kinetics of NFPP. How oxygen inhibition in the near field strongly affects photopolymerization is highlighted, more than in the far field.

Multipixel x ray detection integrated at the end of a narrow multicore fiber.

We introduce and demonstrate the concept of a multipixel detector integrated at the tip of an individual multicore fiber. A pixel consists here of an aluminum-coated polymer microtip incorporating a scintillating powder. Upon irradiation, the luminescence released by the scintillators is efficiently transferred into the fiber cores owing to the specifically elongated metal-coated tips that ensure efficient luminescence matching to the fiber modes. With each pixel being selectively coupled to one of the cores of the multicore optical fiber, the resulting fiber-integrated x ray detection process is totally free from inter-pixel cross talk. Our approach holds promise for fiber-integrated probes and cameras for remote x and gamma ray analysis and imaging in hard-to-reach environments.

Synthesis and Integration of Hybrid Metal Nanoparticles Covered with a Molecularly Imprinted Polymer Nanolayer by Photopolymerization.

Interfacing recognition materials with transducers has consistently presented a challenge in the development of sensitive and specific chemical sensors. In this context, a method based on near-field photopolymerization is proposed to functionalize gold nanoparticles, which are prepared by a very simple process. This method allows in situ preparation of a molecularly imprinted polymer for sensing by surface-enhanced Raman scattering (SERS). In a few seconds, a functional nanoscale layer is deposited by photopolymerization on the nanoparticles. In this study, the dye Rhodamine 6G was chosen as a model target molecule to demonstrate the principle of the method. The detection limit is 500 pM. Due to the nanometric thickness, the response is fast, and the substrates are robust, allowing regeneration and reuse with the same performance level. Finally, this method of manufacturing has been shown to be compatible with integration processes, allowing the future development of sensors integrated in microfluidic circuits and on optical fibers.

Deep-UV laser direct writing of photoluminescent ZnO submicron patterns: an example of nanoarchitectonics concept.

Micro- and nanopatterning of metal oxide materials is an important process to develop electronic or optoelectronic devices. ZnO is a material of choice for its semiconducting and photoluminescence properties. In the frame of the nanoarchitectonics concept, we have developed and investigated a new process that relies on direct writing laser patterning in the Deep-UV (DUV) range to prepare photoluminescent microstructures of ZnO at room temperature, under air. This process is based on a synthesis of colloidal ZnO nanocrystals (NCs) with a careful choice of the ligands on the surface to obtain an optimal (i) stability of the colloids, (ii) redissolution of the non-insolated parts and (iii) cross-linking of the DUV-insolated parts. The mechanisms of photocrosslinking are studied by different spectroscopic methods. This room temperature process preserves the photoluminescence properties of the NCs and the wavelength used in DUV allows to reach a sub-micrometer resolution, which opens new perspectives for the integration of microstructures on flexible substrates for optoelectronic applications.

Two-Photon Absorption Cooperative Effects within Multi-Dipolar Ruthenium Complexes: The Decisive Influence of Charge Transfers.

One- and two-photon characterizations of a series of hetero- and homoleptic [RuL3-n(bpy)n]2+ (n = 0, 1, 2) complexes carrying bipyridine π-extended ligands (L), have been carried out. These π-extended D-π-A-A-π-D-type ligands (L), where the electron donor units (D) are based on diphenylamine, carbazolyl, or fluorenyl units, have been designed to modulate the conjugation extension and the donating effect. Density functional theory calculations were performed in order to rationalize the observed spectra. Calculations show that the electronic structure of the π-extended ligands has a pronounced effect on the composition of HOMO and LUMO and on the metallic contribution to frontier MOs, resulting in strikingly different nonlinear properties. This work demonstrates that ILCT transitions are the keystone of one- and two-photon absorption bands in the studied systems and reveals how much MLCT and LLCT charge transfers play a decisive role on the two-photon properties of both hetero- and homoleptic ruthenium complexes through cooperative or suppressive effects.

Thionolactone as a Resin Additive to Prepare (Bio)degradable 3D Objects via VAT Photopolymerization.

Three-dimensional (3D) printing and especially VAT photopolymerization leads to cross-linked materials with high thermal, chemical, and mechanical stability. Nevertheless, these properties are incompatible with requirements of degradability and re/upcyclability. We show here that thionolactone and in particular dibenzo[c,e]-oxepane-5-thione (DOT) can be used as an additive (2 wt %) to acrylate-based resins to introduce weak bonds into the network via a radical ring-opening polymerization process. The low amount of additive makes it possible to modify the printability of the resin only slightly, keep its resolution intact, and maintain the mechanical properties of the 3D object. The resin with additive was used in UV microfabrication and two-photon stereolithography setups and commercial 3D printers. The fabricated objects were shown to degrade in basic solvent as well in a homemade compost. The rate of degradation is nonetheless dependent on the size of the object. This feature was used to prepare 3D objects with support structures that could be easily solubilized.

Light-Sensitive Alkoxyamines as Versatile Spatially- and Temporally- Controlled Precursors of Alkyl Radicals and Nitroxides.

The use of UV/visible light irradiation as a means to initiate organic syntheses is increasingly attractive due to the high spatial and temporal control conferred by photochemical processes. The aim of this work is thus to demonstrate that alkoxyamines bearing a chromophore on the alkyl moiety can provide a photoprotecting group for the sensitive nitroxide functionality, that is known to degrade through redox processes. The dissociation of various photosensitive alkoxyamines was studied from 223 to 298 K under UV/visible irradiation, depending on the nature of the chromophore. In each case a rapid (typically in less than 1 h) and near-quantitative dissociation was observed. As an illustration of the interest of this approach, a pyrene-based alkoxyamine was employed for the spatially controlled coupling of polymer chains onto Si wafers to produce micropatterned surfaces.

Fabrication of Multimode-Single Mode Polymer Fiber Tweezers for Single Cell Trapping and Identification with Improved Performance.

Optical fiber tweezers have been gaining prominence in several applications in Biology and Medicine. Due to their outstanding focusing abilities, they are able to trap and manipulate microparticles, including cells, needing any physical contact and with a low degree of invasiveness to the trapped cell. Recently, we proposed a fiber tweezer configuration based on a polymeric micro-lens on the top of a single mode fiber, obtained by a self-guided photopolymerization process. This configuration is able to both trap and identify the target through the analysis of short-term portions of the back-scattered signal. In this paper, we propose a variant of this fabrication method, capable of producing more robust fiber tips, which produce stronger trapping effects on targets by as much as two to ten fold. These novel lenses maintain the capability of distinguish the different classes of trapped particles based on the back-scattered signal. This novel fabrication method consists in the introduction of a multi mode fiber section on the tip of a single mode (SM) fiber. A detailed description of how relevant fabrication parameters such as the length of the multi mode section and the photopolymerization laser power can be tuned for different purposes (e.g., microparticles trapping only, simultaneous trapping and sensing) is also provided, based on both experimental and theoretical evidences.

UV-Induced Micropatterning of Complex Functional Surfaces by Photopolymerization Controlled by Alkoxyamines.

We report on the use of an alkoxyamine (AA) for fabrication of functional micropatterns with complex structures by UV mask lithography. The living character of the polymer surface and the vertical spatial control of the repolymerization reaction from few tens of nanometers to few micrometers were demonstrated. The impact of the main parameters governing the controlled polymerization and the reinitiation process activated by light or heat was investigated. Micropatterning is shown to be a powerful method to investigate the physicochemical molecular phenomena. It is possible to control the polymer microstructure thickness from few tens of nanometers to few micrometers. In the last section, some applications are provided showing the potential of the AA for generating covalently bonded hydrophilic/hydrophobic micropatterns or luminescent surfaces. This demonstrates the high versatility and interest of this route.

Sol-Gel TiO2 Thin Film on Au Nanoparticles for Heterogeneous Plasmonic Photocatalysis.

A new, simple method for preparing substrates for photocatalytic applications under visible light is presented. It is based on the preparation of a dense array of gold nanoparticles (AuNPs) by thermal dewetting of a thin gold film followed by spin-coating of a thin TiO2 film prepared by sol-gel chemistry. The photocatalytic properties of these nanocomposite films are studied by surface-enhanced Raman spectroscopy (SERS) following the N-demethylation reaction of methylene blue as a model reaction. This approach shows that the semiconducting layer on the AuNPs can significantly increase the efficiency of the photoinduced reaction. The SERS study also illustrates the influence of parameters such as TiO2 thickness and position (on or under the AuNPs). Ultimately, this study emphasizes that the primary mechanism behind the N-demethylation reaction is both the increase in extinction and the improved electron transfer facilitated by the semiconducting layer. On the other hand, exclusive reliance on photothermal effects is ruled out.

Local reorganization of diblock copolymer domains in directed self-assembly monitored by in situ high-temperature AFM.

In situ high-temperature AFM was used to locally follow dynamic processes, leading to directed self-assembly of copolymers in the context of graphoepitaxy. We focused on the effect of heating for temperatures much higher than the Tg of the used PS-b-PMMA polymer. We showed that such conditions favors the block rearrangement, leading to very regular and perfectly aligned structures in limited times. The use of in situ AFM allowed us to locally investigate the self-organization process at high temperature, thus bringing new insights into self-assembly of block copolymers by graphoepitaxy. In particular, we demonstrate that a slight increase of temperature between 180 and 200 °C allowed overpassing an energy barrier and considerably improves the long distance arrangement, even for relatively short times.

Optical Fiber-Based Polymer Microcantilever for Chemical Sensing: A Through-Fiber Fabrication Scheme.

Fiber optics offer an emerging platform for chemical and biological sensors when engineered with appropriate materials. However, the large aspect ratio makes the optical fiber a rather challenging substrate for standard microfabrication techniques. In this work, the cleaved end of an optical fiber is used as a fabrication platform for cantilever sensors based on functional polymers. The through-fiber fabrication process is triggered by photo-initiated free-radical polymerization and results in a high-aspect-ratio polymer beam in a single step. The dynamic mode application of these cantilevers is first demonstrated in air. These cantilevers are then tuned for sensing applications, including humidity and chemical sensing based on molecularly imprinted polymers.

Sensing Approaches Exploiting Molecularly Imprinted Nanoparticles and Lossy Mode Resonance in Polymer Optical Fibers.

In this work, two different lossy mode resonance (LMR) platforms based on plastic optical fibers (POFs) are developed and tested in a biochemical sensing scenario. The LMR platforms are based on the combination of two metal oxides (MOs), i.e., zirconium oxide (ZrO2) and titanium oxide (TiO2), and deposited on the exposed core of D-shaped POF chips. More specifically, two experimental sensor configurations were obtained by swapping the mutual position of the Mos films over to the core of the D-shaped POF probe. The POF-LMR sensors were first characterized as refractometers, proving the bulk sensitivities. Then, both the POF-LMR platforms were functionalized using molecularly imprinted nanoparticles (nanoMIPs) specific for human transferrin (HTR) in order to carry out binding tests. The achieved results report a bulk sensitivity equal to about 148 nm/RIU in the best sensor configuration, namely the POF-TiO2-ZrO2. In contrast, both optical configurations combined with nanoMIPs showed an ultra-low detection limit (fM), demonstrating excellent efficiency of the used receptor (nanoMIPs) and paving the way to disposable POF-LMR biochemical sensors that are easy-to-use, low-cost, and highly sensitive.

Soft molecularly imprinted nanoparticles with simultaneous lossy mode and surface plasmon multi-resonances for femtomolar sensing of serum transferrin protein.

The simultaneous interrogation of both lossy mode (LMR) and surface plasmon (SPR) resonances was herein exploited for the first time to devise a sensor in combination with soft molecularly imprinting of nanoparticles (nanoMIPs), specifically entailed of the selectivity towards the protein biomarker human serum transferrin (HTR). Two distinct metal-oxide bilayers, i.e. TiO2-ZrO2 and ZrO2-TiO2, were used in the SPR-LMR sensing platforms. The responses to binding of the target protein HTR of both sensing configurations (TiO2-ZrO2-Au-nanoMIPs, ZrO2-TiO2-Au-nanoMIPs) showed femtomolar HTR detection, LODs of tens of fM and KDapp ~ 30 fM. Selectivity for HTR was demonstrated. The SPR interrogation was more efficient for the ZrO2-TiO2-Au-nanoMIPs configuration (sensitivity at low concentrations, S = 0.108 nm/fM) than for the TiO2-ZrO2-Au-nanoMIPs one (S = 0.061 nm/fM); while LMR was more efficient for TiO2-ZrO2-Au-nanoMIPs (S = 0.396 nm/fM) than for ZrO2-TiO2-Au-nanoMIPs (S = 0.177 nm/fM). The simultaneous resonance monitoring is advantageous for point of care determinations, both in terms of measurement's redundancy, that enables the cross-control of the measure and the optimization of the detection, by exploiting the individual characteristics of each resonance.

SnO2-Based Ultra-Flexible Humidity/Respiratory Sensor for Analysis of Human Breath.

Developing ultraflexible sensors using metal oxides is challenging due to the high-temperature annealing step in the fabrication process. Here, we demonstrate the ultraflexible relative humidity (RH) sensor on food plastic wrap by using 808 nm near-infrared (NIR) laser annealing for 1 min at a low temperature (26.2-40.8 °C). The wettability of plastic wraps coated with sol-gel solution is modulated to obtain uniform films. The surface morphology, local temperature, and electrical properties of the SnO2 resistor under NIR laser irradiation with a power of 16, 33, and 84 W/cm2 are investigated. The optimal device can detect wide-range RH from 15% to 70% with small incremental changes (0.1-2.2%). X-ray photoelectron spectroscopy reveals the relation between the surface binding condition and sensing response. Finally, the proposed sensor is attached onto the face mask to analyze the real-time human breath pattern in slow, normal, and fast modes, showing potential in wearable electronics or respiration monitoring.

Direct nanopatterning of 100 nm metal oxide periodic structures by Deep-UV immersion lithography.

Deep-UV lithography using high-efficiency phase mask has been developed to print 100 nm period grating on sol-gel based thin layer. High efficiency phase mask has been designed to produce a high-contrast interferogram (periodic fringes) under water immersion conditions for 244 nm laser. The demonstration has been applied to a new developed immersion-compatible sol-gel layer. A sol-gel photoresist prepared from zirconium alkoxides caped with methacrylic acids was developed to achieve 50 nm resolution in a single step exposure. The nanostructures can be thermally annealed into ZrO(2). Such route considerably simplifies the process for elaborating nanopatterned surfaces of transition metal oxides, and opens new routes for integrating materials of interest for applications in the field of photocatalysis, photovoltaic, optics, photonics or microelectronics.

Micropatterning of electrochemiluminescent polymers based on multipolar Ru-complex two-photon initiators.

In this work, we present an original stereolithography strategy based on multibranched Ru-complexes with a high two-photon initiating ability allowing the 'one-pot' direct laser writing of ECL-active materials deposited onto electro-active surfaces at the µm scale.

Hafnium Oxide Nanostructured Thin Films: Electrophoretic Deposition Process and DUV Photolithography Patterning.

In the frame of the nanoarchitectonic concept, the objective of this study was to develop simple and easy methods to ensure the preparation of polymorphic HfO2 thin film materials (<200 nm) having the best balance of patterning potential, reproducibility and stability to be used in optical, sensing or electronic fields. The nanostructured HfO2 thin films with micropatterns or continuous morphologies were synthesized by two different methods, i.e., the micropatterning of sol-gel solutions by deep ultraviolet (DUV) photolithography or the electrophoretic deposition (EPD) of HfO2 nanoparticles (HfO2-NPs). Amorphous and monoclinic HfO2 micropatterned nanostructured thin films (HfO2-DUV) were prepared by using a sol-gel solution precursor (HfO2-SG) and spin-coating process following by DUV photolithography, whereas continuous and dense monoclinic HfO2 nanostructured thin films (HfO2-EPD) were prepared by the direct EPD of HfO2-NPs. The HfO2-NPs were prepared by a hydrothermal route and studied through the changing aging temperature, pH and reaction time parameters to produce nanocrystalline particles. Subsequently, based on the colloidal stability study, suspensions of the monoclinic HfO2-NPs with morphologies near spherical, spindle- and rice-like shapes were used to prepare HfO2-EPD thin films on conductive indium-tin oxide-coated glass substrates. Morphology, composition and crystallinity of the HfO2-NPs and thin films were investigated by powder and grazing incidence X-ray diffraction, scanning electron microscopy, transmission electron microscopy and UV-visible spectrophotometry. The EPD and DUV photolithography performances were explored and, in this study, it was clearly demonstrated that these two complementary methods are suitable, simple and effective processes to prepare controllable and tunable HfO2 nanostructures as with homogeneous, dense or micropatterned structures.

Plasmon-based free-radical photopolymerization: effect of diffusion on nanolithography processes.

This Article interrogates the mechanisms responsible for nanoscale photopolymerization induced by confined and enhanced electromagnetic fields. Surface plasmon dipolar resonance of individual Ag nanoparticles was used as an optical near-field source to locally trigger the reaction of a photopolymerizable formulation. Laser excitation of the nanoparticles embedded in the formulation reproducibly generates polymer features with typical dimensions ranging from 2 nm to a few tens of nanometer. We have determined the physicochemical parameters and mechanisms controlling the spatial extent of the photopolymerization process. We found that the diffusion of the dye is the main process limiting the polymerization reaction, as opposed to what is observed at the microscale with an equivalent chemical system. This approach demonstrates that plasmon-based polymerization can achieve true nanometer scale resolution and also provides a unique opportunity to investigate photochemistry at this length scale.

Multicore fiber for cold-atomic cloud monitoring.

Thanks to an all solid core photonic crystal fiber (PCF) used as a multicore fiber, we propose and experimentally demonstrate what is to our knowledge a new optical detection scheme for the spontaneous emission collection of cold atoms. A Magneto-Optical Trap (MOT) is placed in front of a polished PCF end-face. As they display a higher optical index than the surrounding cladding silica, the 108 rods (equivalent to a 108 pixels camera) of this PCF are light guiding and behave like an array of detectors. Both global and local properties of the trapped atoms are probed. A MOT lifetime is reported. We also take advantage of the multi-core geometry for a real time detection of the center-of-mass motion of the atomic cloud.