Photoinduced Tetrazole‐Based Functionalization of Off‐Stoichiometric Clickable Microparticles

We report the preparation of tetrazole‐containing step‐growth microparticles and the subsequent use of photoinduced nitrile imine‐mediated tetrazole‐ene cycloaddition (NITEC) reactions on the particles with spatiotemporal control. Microparticles with an average diameter of 4.1 µm and with inherent tetrazole‐ene dual functionality are prepared by a one‐pot off‐stoichiometric thiol‐Michael addition dispersion polymerization. The NITEC reaction is performed efficiently in the solid phase by UV irradiation, leading to the formation of fluorescent pyrozoline adducts, with an estimated quantum yield of 0.7. Particle concentration‐independent reaction kinetics are observed and full conversion is reached within 10 min of UV exposure at an intensity of 8 mW cm^?2. Temporal control is demonstrated with either UV or rooftop sunlight irradiation of variable duration. By using two‐photon writing with a laser centered around 700 nm wavelength, spatial control is demonstrated with micrometer‐scale resolution via surface patterning of the microparticles. Further, microparticles with exclusive tetrazole functionality are prepared by a one‐pot, two‐step thiol‐Michael addition dispersion polymerization. The NITEC reaction between tetrazole‐functional particles and acrylates in solution is examined at various tetrazole/alkene molar ratios, and a 10:1 excess of alkenes in solution is found necessary for efficient functionalization.     

Photoclickable Surfaces for Profluorescent Covalent Polymer Coatings

The nitrile imine‐mediated tetrazole‐ene cycloaddition reaction (NITEC) is introduced as a powerful and versatile conjugation tool to covalently ligate macromolecules onto variable (bio)surfaces. The NITEC approach is initiated by UV irradiation and proceeds rapidly at ambient temperature yielding a highly fluorescent linkage. Initially, the formation of block copolymers by the NITEC methodology is studied to evidence its efficacy as a macromolecular conjugation tool. The grafting of polymers onto inorganic (silicon) and bioorganic (cellulose) surfaces is subsequently carried out employing the optimized reaction conditions obtained from the macromolecular ligation experiments and evidenced by surface characterization techniques, including X‐ray photoelectron spectroscopy and FT‐IR microscopy. In addition, the patterned immobilization of variable polymer chains onto profluorescent cellulose is achieved through a simple masking process during the irradiation.     

TiO2 Nanoparticle–Photopolymer Composites for Volume Holographic Recording

A new and efficient photopolymer for the recording of volume holograms is presented. The material comprises a mixture of UV‐sensitive acrylates and grafted titanium dioxide nanoparticles with an average size of 4 nm. We report the formation of holographic gratings with refractive‐index modulation amplitudes of up to 15.5 × 10^–3—an improvement of more than a factor of four over the base material without nanoparticles—while maintaining a low level of scattering and a high transparency in the visible‐wavelength range. The influence of the composition of the acrylate system on the final properties of the holographic material is also investigated and discussed. The presence of multifunctional monomers favors the compositional segregation of the different components, while the addition of monofunctional acrylate, highly compatible with the grafting of the nanoparticles, favors the dilution of these nanoparticles.     

Cover Picture: TiO2 Nanoparticle–Photopolymer Composites for Volume Holographic Recording (Adv. Funct. Mater. 10/2005)

TiO2 nanoparticle–photopolymer composites have been employed for volume holographic recording, as reported by Sánchez and co‐workers on p. 1623. Photoinduced segregation of the high refractive index, grafted nanoparticles between polymer‐rich areas leads to improved refractive‐index modulation amplitudes with respect to the base material without nanoparticles. The cover schematically shows a holographic grating registered in this nanocomposite material. These nanocomposite materials should enable the production of holographic optical elements to efficiently control light with angle and wavelength selectivity. This could be used, for example, in liquid‐crystal display technology. A new and efficient photopolymer for the recording of volume holograms is presented. The material comprises a mixture of UV‐sensitive acrylates and grafted titanium dioxide nanoparticles with an average size of 4 nm. We report the formation of holographic gratings with refractive‐index modulation amplitudes of up to 15.5 × 10^–3—an improvement of more than a factor of four over the base material without nanoparticles—while maintaining a low level of scattering and a high transparency in the visible‐wavelength range. The influence of the composition of the acrylate system on the final properties of the holographic material is also investigated and discussed. The presence of multifunctional monomers favors the compositional segregation of the different components, while the addition of monofunctional acrylate, highly compatible with the grafting of the nanoparticles, favors the dilution of these nanoparticles.     

Decoupling Theranostics with Rare Earth Doped Nanoparticles

Theranostic nanoagents targeted for personalized medicine provide a unified platform for therapeutics and diagnostics. To be able to discretely control each individually, allows for safer, more precise, and truly multifunctional theranostics. Rare earth doped nanoparticles can be rationally tailored to best match this condition with the aid of core/shell engineering. In such nanoparticles, the light‐mediated theranostic approach is functionally decoupled—therapeutics or diagnostics are prompted on‐demand, by wavelength‐specific excitation. These decoupled rare earth nanoparticles (dNPs) operate entirely under near‐infrared (NIR) excitation, for minimized light interference with the target and extended tissue depth action. Under heating‐free 806 nm irradiation, dNPs behave solely as high‐contrast NIR‐to‐NIR optical markers and nanothermometers, visualizing and probing the area of interest without prompting the therapeutic effect beforehand. On the contrary, 980 nm NIR irradiation is upconverted by the dNPs to UV/visible light, which triggers secondary photochemical processes, e.g., generation of reactive oxygen species by photosensitizers coupled to the dNPs, causing damage to cancer cells. Additionally, integration of NIR nanothermometry helps to control the temperature in the vicinity of the dNPs avoiding possible overheating and quenching of upconversion (UC) emission, harnessed for photodynamic therapy. Overall, a new direction is outlined in the development of state‐of‐the‐art rare earth based theranostic nanoplatforms.     

CdS‐Nanoparticle/Polymer Composite Shells Grown on Silica Nanospheres by Atom‐Transfer Radical Polymerization

In this paper we describe the combined use of surface‐initiated atom transfer radical polymerization (ATRP) and a gas/solid reaction in the direct preparation of CdS‐nanoparticle/block‐copolymer composite shells on silica nanospheres. The block copolymer, consisting of poly(cadmium dimethacrylate) (PCDMA) and poly(methyl methacrylate) (PMMA), is obtained by repeatedly performing the surface‐initiated ATRP procedures in N,N‐dimethylformamide (DMF) solution at room temperature, using cadmium dimethacrylate (CDMA) and methyl methacrylate (MMA) as the monomers. CdS nanoparticles with an average size of about 3 nm are generated in situ by exposing the silica nanospheres coated with block‐copolymer shells to H₂S gas. These synthetic core–shell nanospheres were characterized using transmission electron microscopy (TEM), dynamic light scattering (DLS), thermogravimetric analysis (TGA), diffuse reflectance UV‐vis spectroscopy, X‐ray photoelectron spectroscopy (XPS), and powder X‐ray diffraction (XRD). These composite nanospheres exhibit strong red photoluminescence in the solid state at room temperature.     

Photochemical Generation of Light Responsive Surfaces

The preparation of patterned photoswitchable surfaces by employing the nitrile imine‐mediated tetrazole ene cycloaddition (NITEC) photoinduced reaction in the presence of dipolarophiles based on photoresponsive azobenzene moieties is reported. The dipolarophile used is a maleimide carrying either an azobenzene unit or a first generation dendron containing two azobenzene units. X‐ray photoelectron spectroscopy (XPS) is employed to analyze the functionalized silicon wafers, while time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) evidences the spatial control of the functionalization of the surface achieved by using a micropatterned shadow mask. Water contact angle measurements and optical inspection observing the behavior of a water droplet demonstrate the photoinduced change on wettability of the structured functionalized surfaces due to the reversible trans‐to‐cis isomerization of the azobenzene moities.     

Photopatterned Multidimensional Fluorescent Images

New methods that yield covert fluorescent images are of significant interest for applications in anti‐counterfeit technology. Printing methods that offer access to spatially controlled fluorescence intensity are needed in order to accurately reproduce unique and complex images. Herein, the use of photoreactive inks containing 9,9′‐bis(anthracene)sulfoxide (AnSO) to create complex images with spatially controlled fluorescence intensity is presented. Under UV irradiation, the SO‐bridge between anthracene units in AnSO is extruded to yield the highly luminescent molecule 9,9′‐bianthryl (BA) in quantitative yields. The irreversible formation of BA is leveraged to create multidimensional fluorescent security features that can be patterned using light and easily interpreted using the CCD camera of a mobile phone.     

Biodegradable Fluorescent Nanoparticles for Endoscopic Detection of Colorectal Carcinogenesis

Early and comprehensive endoscopic detection of colonic dysplasia—the most clinically significant precursor lesion to colorectal adenocarcinoma—provides an opportunity for timely, minimally invasive intervention to prevent malignant transformation. Here, the development and evaluation of biodegradable near‐infrared fluorescent silica nanoparticles (FSN) that have the potential to improve adenoma detection during fluorescence‐assisted white‐light colonoscopic surveillance in rodent and human‐scale models of colorectal carcinogenesis is described. FSNs are biodegradable (t_1/2 of 2.7 weeks), well‐tolerated, and enable detection and delineation of adenomas as small as 0.5 mm² with high tumor‐to‐background ratios. Furthermore, in the human scale, APC^1311/+ porcine model, the clinical feasibility and benefit of using FSN‐guided detection of colorectal adenomas using video‐rate fluorescence‐assisted white‐light endoscopy is demonstrated. Since nanoparticles of similar size (e.g., 100–150 nm) or composition (i.e., silica and silica/gold hybrid) have already been successfully translated to the clinic, and clinical fluorescent/white‐light endoscopy systems are becoming more readily available, there is a viable path towards clinical translation of the proposed strategy for early colorectal cancer detection and prevention in high‐risk patients.     

Influence of Side‐Chain Structure and Irradiation Condition on Photoalignment of Ladder‐Like Polysiloxane Films

In this paper we consider the photo‐induced aligning capability of various ladder‐like polysiloxane‐based photoalignment films—which could be used in liquid‐crystal displays—bearing different photoreactive side chains, i.e., laterally grafted cinnamate/azobenzene‐based dual photoreactive side chains with a short or longer spacer, and terminally fixed coumarin‐containing side chains. Results from polarized optical microscopy (POM), Fourier‐transform infrared (FTIR) spectroscopy, surface‐enhanced Raman scattering (SERS), atomic force microscopy (AFM), etc., are integrated to elucidate the influence of side‐chain structure and the irradiation conditions on the photoalignment of ladder‐like polysiloxane films. It is demonstrated that the film containing the dual photoreactive group with a longer spacer exhibits better alignment properties. Reasonably, the concerted photoreactions of the dual photoreactive group and the longer spacer are beneficial to the cooperative motion of chromophores at the “domain level”, resulting in improved alignment facility and stability. The complicated effects of irradiation conditions and moderate annealing are also discussed. High‐quality alignment of the polysilsesquioxane (LPS)‐based photoalignment film LPS‐CA11 with a longer spacer between the LPS main chain and cinnamoyl/azobenzene side chains can be achieved only within an optimal range of exposure (5–8 J cm^–2), while the pretilt angles can be adjusted in the range 0.5°–7° by varying the incident light intensity. Additionally, moderate annealing before and after illumination can markedly improve the alignment uniformity by self‐healing of defects.     

Densification of Oxide Nanoparticle Thin Films by Irradiation with Visible Light

A technique is presented that allows for altering of the physical characteristics of films of TiO₂ nanoparticles by exposure to visible light. In this technique, dye‐sensitized oxide nanoparticles are deposited on a substrate by dip‐coating. Photodissociation of the organic ligand layer leads to cross‐linking of the nanoparticles. Consequently, irradiated films have a decreased porosity, an increased index of refraction and an increased hydrophobicity. Films irradiated with green light are compared to films irradiated with UV light. Within experimental error, visible‐ and UV‐illumination induces the same changes in the films. The mechanism of surfactant elimination in dye‐sensitized oxide particles is discussed, patterning is demonstrated, and prospective applications of the technique are considered.     

Broadband Photoresponse Enhancement of a High‐Performance t‐Se Microtube Photodetector by Plasmonic Metallic Nanoparticles

Broadband responsivity enhancement of single Se microtube (Se‐MT) photodetectors in the UV–visible region is presented in this research. The pristine Se‐MT photodetector demonstrates broadband photoresponse from 300 to 700 nm with peak responsivity of ≈19 mA W^?1 at 610 nm and fast speed (rise time 0.32 ms and fall time 23.02 ms). To further enhance the responsivity of the single Se‐MT photodetector, Au and Pt nanoparticles (NPs) are sputtered on these devices. In contrast to only enhancement of responsivity in UV region by Pt NPs, broadband responsivity enhancement (≈600% to ≈800%) of the Se‐MT photodetector is realized from 300 to 700 nm by tuning the size and density of Au NPs. The broadband responsivity enhancement phenomena are interpreted by both the surface modification and surface plasmon coupling. The experimental results of this work provide an additional opportunity for fabricating high‐performance UV–visible broadband photodetectors.     

Synthesis of Amphiphilic Copolymers of Poly(ethylene oxide) and Poly(ϵ‐caprolactone) with Different Architectures,and Their Role in the Preparation of Stealthy Nanoparticles

Well‐defined copolymers of biocompatible poly(?‐caprolactone) (PCL) and poly(ethylene oxide) (PEO) are synthesized by two methods. Graft copolymers with a gradient structure are prepared by ring‐opening copolymerization of ?‐caprolactone (?CL) with a PEO macromonomer of the ?CL‐type. The ?CL polymerization is initiated by a PEO macroinitiator to prepare diblock copolymers. These amphiphilic copolymers are used as stabilizers for biodegradable poly(D,L ‐lactide) (PLA) nanoparticles prepared by a nanoprecipitation technique. The effect of the copolymer characteristic features (architecture, composition, and amount) on the nanoparticle formation and structure is investigated. The average size, size distribution, and stability of aqueous suspensions of the nanoparticles is measured by dynamic light scattering. For comparison, an amphiphilic random copolymer, poly(methyl methacrylate‐co‐methacrylic acid) (P(MMA‐co‐MA)), is synthesized. The stealthiness of the nanoparticles is analyzed in relation to the copolymer used as stabilizer. For this purpose, the activation of the complement system by nanoparticles is investigated in vitro using human serum. This activation is much less important whenever the nanoparticles are stabilized by a PEO‐containing copolymer rather than by the P(MMA‐co‐MA) amphiphile. The graft copolymers with a gradient structure and the diblock copolymers with similar macromolecular characteristics (molecular weight and hydrophilicity) are compared on the basis of their capacity to coat PLA nanoparticles and to make them stealthy.     

Photopatternable Reflective Films Produced by Nanolayer Extrusion

Novel patternable, light‐reflecting multilayer polymer films are presented. The investigated elements comprise 1024 nanolayers of two different, transparent polymers in strictly alternating fashion. Polymers with different refractive indices were employed and the individual layer thickness was controlled between 50 and 200 nm; as a result the investigated films exhibit pronounced optical interference effects. Different photoreactive additives were integrated into the multilayer films, rendering the optical characteristics of these elements tunable. One approach relied on the use of photoreactive blends of poly(methyl methacrylate) (PMMA) and up to 25 wt.‐% of trans‐cinnamic acid (CA) or trans‐methyl cinnamate (MC). Upon exposure to ultraviolet (UV) radiation, CA and MC undergo dimerization through 2+2 cycloaddition, leading to a significant decrease of the blend's refractive index. As a result, the reflectivity of the multilayer films based on these photoreactive blends changes considerably upon photoreaction. The second approach was based on the use of blends of PMMA and 2‐(2′‐benzoylphenyl)benzoxazole (BzPO), a ‘caged’ photoluminescent dye. This benzoyl ester of 2‐(2′‐hydroxyphenyl)benzoxazole (HPBO) is not photoluminescent, but upon exposure to appropriate UV radiation, the ester bond is cleaved, and the photoluminescent HPBO is quantitatively restored. Thus, using conventional photolithographic techniques, reflective multilayer films were patterned with photoluminescent designs.     

Broadband scattering of the solar spectrum by spherical metal nanoparticles

Metal nanoparticles offer the possibility of improved light trapping in solar cells, but careful design is required to maximise scattering and minimise parasitic absorption across the wavelength range of interest. We present an analysis of the broadband scattering and absorption characteristics of spherical metal nanoparticles, optimized for either crystalline silicon (c‐Si) or amorphous silicon (a‐Si:H) solar cells. A random two‐dimensional array of optimally sized Ag spheres can scatter over 97% of the AM1.5 spectrum from 400 to 1100 nm. Larger particles are required for c‐Si devices than a‐Si:H due to the increased spectral range, with optimum particle sizes ranging from 60 nm for a‐Si:H to 116 nm for c‐Si. Positioning the particles at the rear of the solar cell decreases absorption losses because these principally occur at short wavelengths. Increasing the refractive index of the surrounding medium beyond the optimum value, which is 1.0 for a‐Si:H and 1.6 for c‐Si, shifts absorption to longer wavelengths and decreases scattering at short wavelengths. Ag nanoparticles scatter more of the solar spectrum than Au, Cu or Al nanoparticles. Of these other metals, Al can only be considered for a‐Si:H applications due to high absorption in the near‐infrared, whereas Au and Cu can only be considered for the rear of c‐Si devices due to high absorption in the ultraviolet (UV) and visible. In general, we demonstrate the importance of considering the broadband optical properties of metal nanoparticles for photovoltaic applications. Copyright © 2012 John Wiley & Sons, Ltd.     

Preparation and Organization of Nanoscale Polyelectrolyte‐Coated Gold Nanoparticles

The layer‐by‐layer (LbL) desposition of oppositely charged polyelectrolytes from adsorption solutions of different ionic strength onto ～7 nm diameter carboxylic acid‐derivatized gold nanoparticles has been studied. The polyelectrolyte‐modified nanoparticles were characterized by UV‐vis spectrophotometry, microelectrophoresis, analytical ultracentrifugation, and transmission electron microscopy. UV‐vis data showed that the peak plasmon absorption wavelength of the gold nanoparticles red‐shifted after each adsorption step, and microelectrophoresis experiments revealed a reversal in the surface charge of the nanoparticles following deposition of each layer. These data are consistent with the formation of polyelectrolyte layers on the nanoparticles. Analytical ultracentrifugation showed an increase in mean nanoparticle diameter on adsorption of the polyelectrolytes, confirming the formation of gold‐core/polyelectrolyte‐shell nanoparticles. Transmission electron microscopy studies showed no signs of aggregation of the polyelectrolyte‐coated nanoparticles. The adsorption of the polyelectrolyte‐coated gold nanoparticles onto oppositely charged planar supports has also been examined. UV‐vis spectrophotometry and atomic force microscopy showed increased amounts of nanoparticles were adsorbed with increasing ionic strength of the nanoparticle dispersions. This allows control of the nanoparticle surface loading by varying the salt content in the nanoparticle dispersions used for adsorption. The LbL strategy used in this work is expected to be applicable to other nanoparticles (e.g., semiconductors, phosphors), thus providing a facile means for their controlled surface modification through polyelectrolyte nanolayering. Such nanoparticles are envisaged to have applications in the biomedical and bioanalytical fields, and to be useful building blocks for the creation of advanced nanoparticle‐based films.     

Novel p–p Heterojunctions Self‐Powered Broadband Photodetectors with Ultrafast Speed and High Responsivity

Novel inorganic/organic self‐powered UV–vis photodetectors based on single Se microtube and conducting polymers—polyaniline (PANI), polypyrrole (PPy), and poly(3,4‐ethylenedioxythiophene) (PEDOT)—are fabricated. The conducting polymers are directly coated on the surface of a single Se microtube via a facile and low‐cost in situ polymerization method. The integrated Se/PANI photodetector with 45‐nm‐thick PANI layer shows excellent self‐powered behavior under UV–vis light illumination. In particular, it exhibits high on/off ratio of 1.1 × 10³, responsivity (120 mA W^?1), large detectivity (3.78 × 10¹¹ Jones), and ultrafast response speed (rise time of 4.5 µs and fall time of 2.84 ms) at zero bias at 610 nm (0.434 mW cm^?2)‐light illumination. Moreover, the individual Se/PPy and Se/PEDOT self‐powered photodetectors also exhibit fast and stable responses, including responsivity of 70 and 5.5 mA W^?1, rise time of 0.35 and 1.00 ms, fall time of 16.97 and 9.78 ms, respectively. Given the simple device architecture and low cost fabrication process, this work provides a promising way to fabricate inorganic/organic, high‐performance, self‐powered photodetectors.     

UV‐Light‐Driven Immobilization of Surface‐Functionalized Oxide Nanocrystals onto Silicon

TiO₂ nanorods (NRs) and γ‐Fe₂O₃ nanocrystals (NCs) passivated with unsaturated long‐chain carboxylic acids, namely 10‐undecylenic acid (10UDA) and oleic acid (OLEA), are covalently anchored to Si(100) at room temperature by UV‐light‐driven reaction of hydrogenated silicon with the carbon–carbon double bond (–C?C–) moieties of the capping surfactants. The high reactivity of vinyl groups towards Si provides a general tool for attaching particles of both materials via Si–C bonds. Interestingly, TiO₂ NRs were efficiently attached to silicon even when capped by OLEA. This latter finding has been explained by a photocatalytic mechanism involving the primary role of hydroxyl radicals that can be generated upon bandgap TiO₂ photoexcitation with UV light. The increased oxide coverage achievable on Si opens access to further surface manipulation, as demonstrated by the possibility of depositing an additional film of Au nanoparticles onto TiO₂ via TiO₂‐catalyzed visible‐light‐driven reduction of aqueous AuCl₄^– ions. Extensive morphological and chemical characterization of the obtained NC‐functionalized Si substrates is provided to support the effectiveness of proposed photochemical approaches.     

Photo‐Induced Functionalization of Spherical and Planar Surfaces via Caged Thioaldehyde End‐Functional Polymers

The synthesis and application of a novel reversible addition‐fragmentation chain transfer (RAFT) agent carrying a photocaged thioaldehyde moiety is described (λ_max = 355 nm). RAFT polymerization of styrene, dimethylacrylamide and a glycomonomer is evidenced (3600 g mol^?1 ≤ M_n ≤ 15 000 g mol^?1; 1.07 ≤ ? ≤ 1.20) with excellent end‐group fidelity. The photogenerated thioaldehyde on the chain ends can undergo hetero Diels–Alder reactions with dienes as well as reactions with nucleophiles. The terminal photoreactive polymers are photografted to porous diene‐reactive polymeric microspheres. The grafted particles are in‐depth characterized via scanning electron microscopy, elemental analysis, X‐ray photoelectron spectroscopy, and high resolution FT‐IR microscopy, leading to a qualitative as well as quantitative image of the core–shell objects. Grafting densities up to 0.10 molecules nm^?2 are reached. The versatility of the thioaldehyde ligation is evidenced by spatially resolved grafting of polystyrene onto nucleophilic groups present in poly (dopamine) (PDA)‐coated glass slides and silicon wafers via two‐photon direct laser writing (DLW) imaged by ToF‐SIMS. The combination of thioaldehyde ligation, RAFT polymerization, and DLW allows for the spatially resolved grafting of a vast range of polymers onto various substrates in any desired pattern with sub‐micrometer resolution.     

One‐Step Fabrication of Multifunctional Core‐Shell Fibres by Co‐Electrospinning

In the present study, multifunctional core‐shell fibre mats were designed by co‐electrospinning. These core‐shell fibre mats have three different functionalities: 1) they are magnetic, 2) they change their optical properties with the pH of the media, and 3) they are sensitive to O₂. The shell is formed by a fluorescent pH‐sensitive co‐polymer which was previously synthesised and characterized by our research group. The core is a suspension formed by magnetic nanoparticles in a solution made up by a lipophilic indicator dye (oxygen indicator; PtOEP) and, poly‐methyl methacrylate, in THF. The magnetic nanoparticles were prepared by encapsulation of magnetite within a cross‐linked polymeric matrix (MMA‐co‐EDMA). To our knowledge, this is the first time that three functionalities (magnetic properties, sensitivity to pH, and response to O₂ concentration) were successful conjugated on the same micro‐ or nano‐material via a facile one‐step process with high yield and cost effectiveness. The morphology of the well‐organized core‐shell fibres were characterized by high resolution scanning electron microcopy (HRSEM), transmission electron microcopy (TEM), and confocal laser microscopy. The luminescent properties of core‐shell fibre mats were analysed and successfully used for simultaneously monitoring pH (from 6 to 8) and O₂, showing complete reversibility, high sensitivity (i.e., K_sv = 7.07 bar^?1 for determining O₂ in aqueous media), high magnetic susceptibility, and short response times.