Efficient Photochemical Approaches for Spatially Resolved Surface Functionalization

Optical fiber biosensors toward in vivo detection

This review takes stock of the various optical fiber-based biosensors that could be used for in vivo applications. We discuss the characteristics that biosensors must have to be suitable for such applications and the corresponding transduction modes. In particular, we focus on optical fiber biosensors based on fluorescence, evanescent wave, plasmonics, interferometry, and Raman phenomenon. The operational principles, implemented solutions, and performances are described and debated. The different sensing configurations, such as the side- and tip-based fiber biosensors, are illustrated, and their adaptation for in vivo measurements is discussed. The required implementation of multiplexed biosensing on optical fibers is shown. In particular, the use of multi-fiber assemblies, one of the most optimal configurations for multiplexed detection, is discussed. Different possibilities for multiple localized functionalizations on optical fibers are presented. A final section is devoted to the practical in vivo use of fiber-based biosensors, covering regulatory, sterilization, and packaging aspects. Finally, the trends and required improvements in this promising and emerging field are analyzed and discussed.

Protein-based bioactive coatings: from nanoarchitectonics to applications

Protein-based bioactive coatings have emerged as a versatile and promising strategy for enhancing the performance and biocompatibility of diverse biomedical materials and devices. Through surface modification, these coatings confer novel biofunctional attributes, rendering the material highly bioactive. Their widespread adoption across various domains in recent years underscores their importance. This review systematically elucidates the behavior of protein-based bioactive coatings in organisms and expounds on their underlying mechanisms. Furthermore, it highlights notable advancements in artificial synthesis methodologies and their functional applications in vitro. A focal point is the delineation of assembly strategies employed in crafting protein-based bioactive coatings, which provides a guide for their expansion and sustained implementation. Finally, the current trends, challenges, and future directions of protein-based bioactive coatings are discussed.

Fluorescence-readout as a powerful macromolecular characterisation tool

The last few decades have witnessed significant progress in synthetic macromolecular chemistry, which can provide access to diverse macromolecules with varying structural complexities, topology and functionalities, bringing us closer to the aim of controlling soft matter material properties with molecular precision. To reach this goal, the development of advanced analytical techniques, allowing for micro-, molecular level and real-time investigation, is essential. Due to their appealing features, including high sensitivity, large contrast, fast and real-time response, as well as non-invasive characteristics, fluorescence-based techniques have emerged as a powerful tool for macromolecular characterisation to provide detailed information and give new and deep insights beyond those offered by commonly applied analytical methods. Herein, we critically examine how fluorescence phenomena, principles and techniques can be effectively exploited to characterise macromolecules and soft matter materials and to further unravel their constitution, by highlighting representative examples of recent advances across major areas of polymer and materials science, ranging from polymer molecular weight and conversion, architecture, conformation to polymer self-assembly to surfaces, gels and 3D printing. Finally, we discuss the opportunities for fluorescence-readout to further advance the development of macromolecules, leading to the design of polymers and soft matter materials with pre-determined and adaptable properties.

Controlling differentiation of stem cells via bioactive disordered cues

Ideal bone tissue engineering is to induce bone regeneration through the synergistic integration of biomaterial scaffolds, bone progenitor cells, and bone-forming factors. Biomimetic scaffolds imitate the native extracellular matrix (ECM) and are often utilized in vitro as analogues of the natural ECM to facilitate investigations of cell-ECM interactions and processes. In vivo, the cellular microenvironment has a crucial impact on regulating cell behavior and functions. A PET surface was activated and then functionalized with mimetic peptides to promote human mesenchymal stem cell (hMSC) adhesion and differentiation into an osteogenic lineage. Spray technology was used to randomly micropattern peptides (RGD and BMP-2 mimetic peptides) on the PET surface. The distribution of the peptides grafted on the surface, the roughness of the surfaces and the chemistry of the surfaces in each step of the treatment were ascertained by atomic force microscopy, fluorescence microscopy, time-of-flight secondary ion mass spectrometry, Toluidine Blue O assay, and X-ray photoelectron spectroscopy. Subsequently, cell lineage differentiation was evaluated by quantifying the expression of immunofluorescence markers: osteoblast markers (Runx-2, OPN) and osteocyte markers (E11, DMP1, and SOST). In this article, we hypothesized that a unique combination of bioactive micro/nanopatterns on a polymer surface improves the rate of morphology change and enhances hMSC differentiation. In DMEM, after 14 days, disordered micropatterned surfaces with RGD and BMP-2 led to a higher osteoblast marker expression than surfaces with a homogeneous dual peptide conjugation. Finally, hMSCs cultured in osteogenic differentiation medium (ODM) showed accelerated cell differentiation. In ODM, our results highlighted the expression of osteocyte markers when hMSCs were seeded on PET surfaces with random micropatterns.

Metabolic glycoengineering - exploring glycosylation with bioorthogonal chemistry

Glycans are involved in numerous biological recognition events. Being secondary gene products, their labeling by genetic methods - comparable to GFP labeling of proteins - is not possible. To overcome this limitation, metabolic glycoengineering (MGE, also known as metabolic oligosaccharide engineering, MOE) has been developed. In this approach, cells or organisms are treated with synthetic carbohydrate derivatives that are modified with a chemical reporter group. In the cytosol, the compounds are metabolized and incorporated into newly synthesized glycoconjugates. Subsequently, the reporter groups can be further derivatized in a bioorthogonal ligation reaction. In this way, glycans can be visualized or isolated. Furthermore, diverse targeting strategies have been developed to direct drugs, nanoparticles, or whole cells to a desired location. This review summarizes research in the field of MGE carried out in recent years. After an introduction to the bioorthogonal ligation reactions that have been used in in connection with MGE, an overview on carbohydrate derivatives for MGE is given. The last part of the review focuses on the many applications of MGE starting from mammalian cells to experiments with animals and other organisms.

Facile and Versatile Method for Micropatterning Poly(acrylamide) Hydrogels Using Photocleavable Comonomers

We here present a micropatterning strategy to introduce small molecules and ligands on patterns of arbitrary shapes on the surface of poly(acrylamide)-based hydrogels. The main advantages of the presented approach are the ease of use, the lack of need to prefabricate photomasks, the use of mild UV light and biocompatible bioconjugation chemistries, and the capacity to pattern low-molecular-weight ligands, such as peptides, peptidomimetics, or DNA fragments. To achieve the above, a monomer containing a caged amine (NVOC group) was co-polymerized in the hydrogel network; upon UV light illumination using a commercially available setup, primary amines were locally deprotected and served as reactive groups for further functionalization. Cell patterning on various cell adhesive ligands was demonstrated, with cells responding to a combination of pattern shape and substrate elasticity. The approach is compatible with standard traction force microscopy (TFM) experimentation and can further be extended to reference-free TFM.

Local Surface Chemistry Dynamically Monitored by Quantitative Phase Microscopy

Surface modification by photo grafting constitutes an interesting strategy to prepare functional surfaces. Precision applications, however, demand quantitative methods able to monitor and control the amount and distribution of surface modifications, which is hard to achieve, particularly in operando conditions. In this paper, a label-free, cost-effective, all-optical method based on wavefront sensing which is able to quantitatively track the evolution of grafted layers in real-time, is presented. By positioning a simple thin diffuser in the close vicinity of a camera, the thickness of grafted patterns is directly evaluated with sub-nanometric sensitivity and diffraction-limited lateral resolution. By performing an in-depth kinetic analysis of the local modification of an inert substrate (glass cover slips) through photografting of arydiazonium salts, different growth regimes are characterized and several parameters are estimated, such as the grafting efficiency, density and the apparent refractive index distribution of the resulting grafted layers. Both focused and widefield-grafting can be quantitatively monitored in real time, providing valuable guidelines to maximize functionalization efficiency. The association of a well-characterized versatile photografting reaction with the proposed flexible and sensitive monitoring strategy enables functional surfaces to be prepared, and puts surface micro- to submicro-structuration within the reach of most laboratories.

Photoredox Catalysis for the Fabrication of Water-Repellent Surfaces with Application for Oil/Water Separation

Silanization processes with perfluoroalkyl silanes have been demonstrated to be effective in developing advanced materials with many functional properties, including hydrophobicity, water repellency, and self-cleaning properties. However, practical industrial applications of perfluoroalkyl silanes are limited by their extremely high cost. On the basis of our recent work on photoredox catalysis for amidation with perfluoroalkyl iodides, its application for surface chemical modification on filter paper, as an illustrative example, has been developed and evaluated. Before photocatalytic amidation, the surface is functionalized with amine functional groups by silanization with 3-(trimethoxysilyl)propylamine. All chemically modified surfaces have been fully characterized by attenuated total reflection infrared (ATR-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), and three-dimensional (3D) profiling to confirm the successful silanization and photocatalytic amidation. After surface modification of the filter papers with perfluoroalkanamide, they show high water repellency and hydrophobicity with contact angles over 120°. These filter papers possess high wetting selectivity, which can be used to effectively separate the organic and aqueous biphasic mixtures. The perfluoroalkanamide-modified filter papers can be used for separating organic/aqueous biphasic mixtures over many cycles without lowering the separating efficiency, indicating their reusability and excellent durability.

Light-Triggered Click Chemistry

The merging of click chemistry with discrete photochemical processes has led to the creation of a new class of click reactions, collectively known as photoclick chemistry. These light-triggered click reactions allow the synthesis of diverse organic structures in a rapid and precise manner under mild conditions. Because light offers unparalleled spatiotemporal control over the generation of the reactive intermediates, photoclick chemistry has become an indispensable tool for a wide range of spatially addressable applications including surface functionalization, polymer conjugation and cross-linking, and biomolecular labeling in the native cellular environment. Over the past decade, a growing number of photoclick reactions have been developed, especially those based on the 1,3-dipolar cycloadditions and Diels-Alder reactions owing to their excellent reaction kinetics, selectivity, and biocompatibility. This review summarizes the recent advances in the development of photoclick reactions and their applications in chemical biology and materials science. A particular emphasis is placed on the historical contexts and mechanistic insights into each of the selected reactions. The in-depth discussion presented here should stimulate further development of the field, including the design of new photoactivation modalities, the continuous expansion of λ-orthogonal tandem photoclick chemistry, and the innovative use of these unique tools in bioconjugation and nanomaterial synthesis.

A Novel Photoinduced Ligation Approach for Cross-Linking Polymerization, Polymer Chain-End Functionalization, and Surface Modification Using Benzoyl Azides

Various ligation processes have recently become a powerful tool in synthetic polymer chemistry. Herein, the use of a new photochemical ligation process as a versatile approach for the cross-linking polymerization, functionalization of polymer chain ends, and surface modification of various materials such as silica and graphene oxide, is demonstrated. The process is based on the formation of urethane linkages by the reaction of photochemically in situ generated isocyanates from benzoyl azides with hydroxyl moieties in the presence of organobase, bicyclo[2.2.2]-1,4-diazaoctane (DABCO) under ambient conditions. The intermediates and obtained materials are characterized by NMR, FTIR, TGA, and TEM analyses. It is believed that this simple and efficient ligation process will expand future applications to fabricate complex macromolecular structures, biomaterials, and gels.

Dual-Wavelength Gated oxo-Diels-Alder Photoligation

The control of chemical functionalization with orthogonal light stimuli paves the way toward manipulating materials with unprecedented spatiotemporal resolution. To reach this goal, we herein introduce a photochemical reaction system that enables two-color control of covalent ligation via an oxo-Diels-Alder cycloaddition between two separate light-responsive molecular entities: a UV-activated photocaged diene based on ortho-quinodimethanes and a carbonyl dienophile appended to a diarylethene photoswitch, whose reactivity can be modulated upon illumination with UV and visible light.

Recent Advances in the Generation of Nitrilium Betaine 1,3-Dipoles

Nitrilium betaine 1,3-dipoles are ubiquitous reagents in organic chemistry, with applications ranging from natural product synthesis to materials science. Given the high reactivity of these zwitterionic motifs, they are invariably generated in situ from a suitable precursor, prior to use. This short review summarises the recent progress in the development of modern approaches towards the formation of these 1,3-dipoles, and their applications within a diverse range of fields.

1 Introduction

2 Nitrile Ylides

2.1 2H-Azirine Rearrangement

2.2 Addition of Nitriles to Carbenes

3 Nitrile Imines

3.1 2,5-Tetrazole Thermolysis

3.2 2,5-Tetrazole Photolysis

3.3 Diaryl Sydnone Photolysis

4 Nitrile Oxides

4.1 Hypervalent Iodine

4.2 The Nitroso Radical

4.3 Green Chemistry Approaches

4.4 Other Approaches

5 Conclusions

Transition-Metal-Free Coupling of 1,3-Dipoles and Boronic Acids as a Sustainable Approach to C-C Bond Formation

The need for alternative, complementary approaches to enable C-C bond formation within organic chemistry is an on-going challenge in the area. Of particular relevance are transformations that proceed in the absence of transition-metal reagents. In the current study, we report a comprehensive investigation of the coupling of nitrile imines and aryl boronic acids as an approach towards sustainable C-C bond formation. In situ generation of the highly reactive 1,3-dipole facilitates a Petasis-Mannich-type coupling via a nucleophilic boronate complex. The introduction of hydrazonyl chlorides as a complementary nitrile imine source to the 2,5-tetrazoles previously reported by our laboratory further broadens the scope of the approach. Additionally, we exemplify for the first time the extension of this protocol into another 1,3-dipole, through the synthesis of aryl ketone oximes from aryl boronic acids and nitrile N-oxides.

Polysaccharide-Based pH-Responsive Nanocapsules Prepared with Bio-Orthogonal Chemistry and Their Use as Responsive Delivery Systems

Bio-orthogonal reactions have become an essential tool to prepare biomaterials; for example, in the synthesis of nanocarriers, bio-orthogonal chemistry allows circumventing common obstacles related to the encapsulation of delicate payloads or the occurrence of uncontrolled side reactions, which significantly limit the range of potential payloads to encapsulate. Here, we report a new approach to prepare pH-responsive nanocarriers using dynamic bio-orthogonal chemistry. The reaction between a poly(hydrazide) crosslinker and functionalized polysaccharides was used to form a pH-responsive hydrazone network. The network formation occurred at the interface of aqueous nanodroplets in miniemulsion and led to the production of nanocapsules that were able to encapsulate payloads of different molecular weights. The resulting nanocapsules displayed low cytotoxicity and were able to release the encapsulated payload, in a controlled manner, under mildly acidic conditions.

Grayscale surface patterning using electrophoretic motion through a heterogeneous hydrogel material

Chemical surface patterning can be an incredibly powerful tool in a variety of applications, as it enables precise spatial control over surface properties. But the equipment required to create functional surface patterns-especially "grayscale" patterns where independent control over species placement and density are needed-is often expensive and inaccessible. In this work, we leveraged equipment and methods readily available to many research labs, namely 3D printing and electroblotting, to generate controlled grayscale surface patterns. Three-dimensional-printed molds were used to cast polyacrylamide hydrogels with regions of variable polymer density; regions of low polymer density within the hydrogels served as reservoirs for proteins that were later driven onto a target surface using electrophoresis. This mechanism was used to deposit grayscale patterns of fluorescently labeled proteins, and the fluorescent intensity of these patterns was measured and compared to a theoretical analysis of the deposition mechanism.

One-Pot Suzuki-Hydrogenolysis Protocol for the Modular Synthesis of 2,5-Diaryltetrazoles

2,5-Diaryltetrazoles are a diverse range of compounds of considerable interest within the field of photochemistry as a valuable precursor of the nitrile imine 1,3-dipole. Current literature approaches toward this heterocycle remain unsuitable for the practical synthesis of a library of these derivatives. Herein, we disclose the development of a modular approach toward 2,5-diaryltetrazoles compatible with an array-type protocol, facilitated by a tandem Suzuki-hydrogenolysis approach.

Reconfigurable Surface with Photodefinable Physicochemical Properties for User-Designable Cell Scaffolds

Surfaces with specific topography and chemical composition are quite useful in many applications ranging from functional interfaces to cell incubation scaffolds. Although these surfaces can be easily fabricated by combining topography-construction methods and surface-functionalization strategies, their properties are often static after fabrication or merely switchable between "on" and "off" states. Developing surfaces that can be on-demand regulated are quite important for the generation of smart surfaces for future applications. In this paper, we present a reconfigurable surface with adjustable topography and chemical functionality utilizing the photodynamic feature of the disulfide bond. Structured surfaces, composed of disulfide-cross-linked polymer networks, were prepared by using disulfide-containing methacrylate as the monomer. We show that the topography and chemical functionality of the surface can be on-demand regulated after its fabrication, with 254 and 365 nm UV light, respectively, allowing to "define" the physicochemical properties of the surface using light before the usage. We also demonstrate the application of such surface as a user-designable cell scaffold, that different cell scaffolds can be generated from one original surface with a simple exposure process, to define the desired bioactivity onto every point of the surface and therefore exactly control cell behaviors on the scaffold.

A water-soluble sunlight erasable ink based on [4 + 4] cycloaddition of 9-substituted anthracene

Here we report a water-soluble sunlight erasable ink based on 9-substituted anthracene for applications in data confidentiality or paper reuse.

Light-driven assembly of biocompatible fluorescent chitosan hydrogels with self-healing ability

Nitrile imine-mediated tetrazole-ene cycloaddition (NITEC) was successfully used to cross-link complementary tetrazole and maleimide chitosan derivatives into hydrogel networks using irradiation.

Metal-free C-C bond formation via coupling of nitrile imines and boronic acids

The challenges of developing sustainable methods of carbon-carbon bond formation remains a topic of considerable importance in synthetic chemistry. Capitalizing on the highly reactive nature of the nitrile imine 1,3-dipole, we have developed a novel metal-free coupling of this species with aryl boronic acids. Photochemical generation of a nitrile imine intermediate and trapping with a palette of boronic acids enabled rapid and facile access to a broad library of more than 25 hydrazone derivatives in up to 92% yield, forming a carbon-carbon bond in a metal free fashion. This represents the first reported example of direct reaction between boronic acids and a 1,3-dipole.

Functional Materials for Two-Photon Polymerization in Microfabrication

Direct laser writing methods based on two-photon polymerization (2PP) are powerful tools for the on-demand printing of precise and complex 3D architectures at the micro and nanometer scale. While much progress was made to increase the resolution and the feature size throughout the years, by carefully designing a material, one can confer specific functional properties to the printed structures thus making them appealing for peculiar and novel applications. This Review summarizes the state-of-the-art of functional resins and photoresists used in 2PP, discussing both the range of material functions available and the methods used to prepare them, highlighting advantages and disadvantages of different classes of materials in achieving certain properties.

A Photochemical Ligation System Enabling Solid-Phase Chemiluminescence Read-Out

The peroxyoxalate chemiluminescence (PO-CL) reaction is among the most powerful and versatile techniques for the detection of hydrogen peroxide (H₂ O₂ ) and has been employed in various biological and chemical applications over the past 50 years. However, its two-component nature (peroxyoxalate and fluorophore) limits its use. This contribution introduces an innovative and versatile photochemical platform technology for the synthesis of inherently fluorescent PO probes by exploiting the nitrile imine-mediated tetrazole-ene cycloaddition (NITEC) reaction. In the presence of hydrogen peroxide, the pioneered "2-in-1" molecule emits either yellow or blue light, depending on tetrazole (Tz) structure. Even in the absence of base, the emitted light remains visible and H₂ O₂ could be detected in the nanomolar range. Critically, the PO-Tz can be readily incorporated into polymeric materials. As a first application of this promising material, a tailor-made PO-Tz is grafted on poly(divinylbenzene) (PDVB) particles to enable solid-phase chemiluminescence on microspheres.

Photosensitive Hydrogel Creates Favorable Biologic Niches to Promote Spinal Cord Injury Repair

Photochemistry is considered to be a promising strategy for hydrogels to mimic the complex and dynamic properties of natural extracellular matrix. However, it is seldom applied in 3D tissue engineering and regenerative medicine due to the attenuation of light. In this study, phenyl azide photchemistry and optical fiber technology are first used to localize adhesive protein on the inner surface of the nerve guidance conduit in a 3D hydrogel scaffold. In vitro coculture assay of neural stem cells (NSCs) shows that photoimmobilization of collagen significantly improves the adhesion and survival of NSCs in the conduit, and exhibits synergistic effect with the sustainable release of growth factor. After implantation in transected spinal cord, the optimized hydrogel scaffold is found to improve the locomotion recovery of rats 12 weeks after spinal cord injury (SCI). Histological analysis suggests that the designed hydrogel scaffold provides a favorable biological niche for neuronal regeneration, thus producing directional neuron tissue and promoting the repair of SCI. This study demonstrates a promising hydrogel scaffold for SCI repair and provides the first understanding of the photoimmobilization of adhesive protein in a 3D hydrogel conduit concerning its functions on spinal cord tissue restoration.

Robust Carbon Nitride-Based Thermoset Coatings for Surface Modification and Photochemistry

Herein, the convenient visible light-induced photografting of hydroxyl ethyl methacrylate onto graphitic carbon nitride (g-CN) is described, leading to well-dispersible g-CN-based precursor polymers that can be injected. Mixing with citric acid as the cross-linker and heating leads to stable thermoset coatings. The process is versatile and easy to perform, leading to g-CN-based coatings. Moreover, the coating can be further functionalized/modified via grafting of other polymer chains, and the resulting structure is useful as photocatalytic surface or as photoelectrode.

Heparinized Polyurethane Surface Via a One-Step Photografting Method

Traditional methods using coupling chemistry for surface grafting of heparin onto polyurethane (PU) are disadvantageous due to their generally low efficiency. In order to overcome this problem, a quick one-step photografting method is proposed here. Three heparin derivatives incorporating 0.21, 0.58, and 0.88 wt% pendant aryl azide groups were immobilized onto PU surfaces, leading to similar grafting densities of 1.07, 1.17, and 1.13 μg/cm², respectively, yet with increasing densities of anchoring points. The most negatively charged surface and the maximum binding ability towards antithrombin III were found for the heparinized PU with the lowest amount of aryl azide/anchor sites. Furthermore, decreasing the density of anchoring points was found to inhibit platelet adhesion to a larger extent and to prolong plasma recalcification time, prothrombin time, thrombin time, and activated partial thromboplastin time to a larger extent. This was also found to enhance the bioactivity of immobilized heparin from 22.9% for raw heparin to 36.9%. This could be explained by the enhanced molecular mobility of immobilized heparin when it is more loosely anchored to the PU surface, as well as a higher surface charge.

Self-reporting visible light-induced polymer chain collapse

We introduce a facile photoinduced self-reporting crosslinking methodology for the compaction of polymer chains in highly diluted solution.

Dynamic Nitroxide Functional Materials

A substrate-independent and versatile coating platform for (spatially resolved) surface functionalization, based on nitroxide radical coupling (NRC) reactions and the formation of thermo-labile alkoxyamine functional groups, was introduced. Nitroxide-decorated poly(glycidyl methacrylate) (PGMA) microspheres, obtained through bioinspired copolymer surface deposition using dopamine and a nitroxide functional dopamine derivative as monomers, were conjugated with small functional groups in a rewritable process. Reversible coding of the nitroxide functional microspheres by NRC and decoding through thermal alkoxyamine fission were monitored and characterized by electron paramagnetic resonance (EPR) spectroscopy and X-ray photoelectron spectroscopy (XPS). In addition, this nitroxide coating system was exploited in "grafting-to" polymer surface ligations of poly(methyl methacrylate) (PMMA) and poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) in spatially confined areas. Polymer strands terminated with an Irgacure 2959 (2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone) photoinitiator were obtained through chain-transfer polymerization, and subsequently coupled to nitroxide-immobilized poly(dopamine) (PDA)-coated silicon substrates by using rapid photoclick NRC reactions. Light-driven polymer surface coding was visualized by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and XPS imaging.

Reconfiguring surface functions using visible-light-controlled metal-ligand coordination

Most surfaces are either static or switchable only between “on” and “off” states for a specific application. It is a challenge to develop reconfigurable surfaces that can adapt to rapidly changing environments or applications. Here, we demonstrate fabrication of surfaces that can be reconfigured for user-defined functions using visible-light-controlled Ru–thioether coordination chemistry. We modify substrates with Ru complex Ru-H₂O. To endow a Ru-H₂O-modified substrate with a certain function, a functional thioether ligand is immobilized on the substrate via Ru–thioether coordination. To change the surface function, the immobilized thioether ligand is cleaved from the substrate by visible-light-induced ligand dissociation, and then another thioether ligand with a distinct function is immobilized on the substrate. Different thioethers endow the surface with different functions. Based on this strategy, we rewrite surface patterns, manipulate protein adsorption, and control surface wettability. This strategy enables the fabrication of reconfigurable surfaces with customizable functions on demand.

Configuring surfaces on-demand for desired functionalities is an ongoing challenge. Here, diverse and tailorable modifications of quartz and porous silica surfaces that are rapidly and reversibly switchable by the use of visible light are achieved via ruthenium-thioether coordination.

Photochemistry for Well-Defined Polymers in Aqueous Media: From Fundamentals to Polymer Nanoparticles to Bioconjugates

This review article highlights recent developments in the field of photochemistry and photochemical reversible deactivation radical polymerization applied to aqueous polymerizations. Photochemistry is a topic of significant interest in the fields of organic, polymer, and materials chemistry because it allows challenging reactions to be performed under mild conditions. Aqueous polymerization is of significant interest because water is an environmentally benign solvent, and the use of water enables complex polymer self-assembly and bioconjugation processes to occur. This review focuses on powerful new developments in photochemical aqueous polymerization reactions and their applications to the synthesis of well-defined polymer nano-objects and bioconjugates. It is anticipated that these aqueous photopolymerizations will enable the next generation of self-assembled structures and biohybrid materials to be developed under mild and environmentally friendly conditions.

Surface Functionalization with Carboxylic Acids by Photochemical Microcontact Printing and Tetrazole Chemistry

In this paper, we show that carboxylic acid-functionalized molecules can be patterned by photochemical microcontact printing on tetrazole-terminated self-assembled monolayers. Upon irradiation, tetrazoles eliminate nitrogen to form highly reactive nitrile imines, which can be ligated with several different nucleophiles, carboxylic acids being the most reactive. As a proof of concept, we immobilized trifluoroacetic acid to monitor the reaction with X-ray photoelectron spectroscopy. Moreover, we also immobilized peptides and fabricated carbohydrate-lectin as well as biotin-streptavidin microarrays using this method. Surface-patterning was demonstrated by fluorescence microscopy and time-of-flight secondary ion mass spectrometry.

Folding polymer chains with visible light

A simple and versatile tool for generating fluorescent single chain polymer nanoparticles with visible light.

And There Was Light: Prospects for the Creation of Micro- and Nanostructures through Maskless Photolithography

In photolithographic processes, the light inducing the photochemical reactions is confined to a small volume, which enables direct writing of micro- and nanoscale features onto solid surfaces without the need of a predefined photomask. The direct writing process can be used to generate topographic patterns through photopolymerization or photo-cross-linking or can be employed to use light to generate chemical patterns on the surface with high spatial control, which would make such processes attractive for bioapplications. The prospects of maskless photolithography technologies with a focus on two-photon lithography and scanning-probe-based photochemical processes based on scanning near-field optical microscopy or beam pen lithography are discussed.

Fully degradable protein nanocarriers by orthogonal photoclick tetrazole-ene chemistry for the encapsulation and release

The encapsulation of sensitive drugs into nanocarriers retaining their bioactivity and achieving selective release is a challenging topic in current drug delivery design. Established protocols rely on metal-catalyzed or unspecific reactions to build the (mostly synthetic) vehicles which may inhibit the drug's function. Triggered by light, the mild tetrazole-ene cycloaddition enables us to prepare protein nanocarriers (PNCs) preserving at the same time the bioactivity of the sensitive antitumor and antiviral cargo Resiquimod (R848). This catalyst-free reaction was designed to take place at the interface of aqueous nanodroplets in miniemulsion to produce core-shell PNCs with over 90% encapsulation efficiency and no unwanted drug release over storage for several months. Albumins used herein are major constituents of blood and thus ideal biodegradable natural polymers for the production of such nanocarriers. These protein carriers were taken up by dendritic cells and the intracellular drug release by enzymatic degradation of the protein shell material was proven. Together with the thorough colloidal analysis of the PNCs, their stability in human blood plasma and the detailed protein corona composition, these results underline the high potential of such naturally derived drug delivery vehicles.

Solution-Phase Photochemical Nanopatterning Enabled by High-Refractive-Index Beam Pen Arrays

A high-throughput, solution-based, scanning-probe photochemical nanopatterning approach, which does not require the use of probes with subwavelength apertures, is reported. Specifically, pyramid arrays made from high-refractive-index polymeric materials were constructed and studied as patterning tools in a conventional liquid-phase beam pen lithography experiment. Two versions of the arrays were explored with either metal-coated or metal-free tips. Importantly, light can be channeled through both types of tips and the appropriate solution phase (e.g., H₂O or CH₃OH) and focused on subwavelength regions of a substrate to effect a photoreaction in solution that results in localized patterning of a self-assembled monolayer (SAM)-coated Au thin film substrate. Arrays with as many as 4500 pyramid-shaped probes were used to simultaneously initiate thousands of localized free-radical photoreactions (decomposition of a lithium acylphosphinate photoinitiator in an aqueous solution) that result in oxidative removal of the SAM. The technique is attractive since it allows one to rapidly generate features less than 200 nm in diameter, and the metal-free tips afford more than 10-fold higher intensity than the tips with nanoapertures over a micrometer propagation length. In principle, this mask-free method can be utilized as a versatile tool for performing a wide variety of photochemistries across multiple scales that may be important in high-throughput combinatorial screening applications related to chemistry, biology, and materials science.

Molecular Switch for Sub-Diffraction Laser Lithography by Photoenol Intermediate-State Cis-Trans Isomerization

Recent developments in stimulated-emission depletion (STED) microscopy have led to a step change in the achievable resolution and allowed breaking the diffraction limit by large factors. The core principle is based on a reversible molecular switch, allowing for light-triggered activation and deactivation in combination with a laser focus that incorporates a point or line of zero intensity. In the past years, the concept has been transferred from microscopy to maskless laser lithography, namely direct laser writing (DLW), in order to overcome the diffraction limit for optical lithography. Herein, we propose and experimentally introduce a system that realizes such a molecular switch for lithography. Specifically, the population of intermediate-state photoenol isomers of α-methyl benzaldehydes generated by two-photon absorption at 700 nm fundamental wavelength can be reversibly depleted by simultaneous irradiation at 440 nm, suppressing the subsequent Diels-Alder cycloaddition reaction which constitutes the chemical core of the writing process. We demonstrate the potential of the proposed mechanism for STED-inspired DLW by covalently functionalizing the surface of glass substrates via the photoenol-driven STED-inspired process exploiting reversible photoenol activation with a polymerization initiator. Subsequently, macromolecules are grown from the functionalized areas and the spatially coded glass slides are characterized by atomic-force microscopy. Our approach allows lines with a full-width-at-half-maximum of down to 60 nm and line gratings with a lateral resolution of 100 nm to be written, both surpassing the diffraction limit.

Facile Quenching and Spatial Patterning of Cylooctynes via Strain-Promoted Alkyne-Azide Cycloaddition of Inorganic Azides

Little is known about the reactivity of strain-promoted alkyne-azide cycloaddition (SPAAC) reagents with inorganic azides. We explore the reactions of a variety of popular SPAAC reagents with sodium azide and hydrozoic acid. We find that the reactions proceed in water at rates comparable to those with organic azides, yielding in all cases a triazole adduct. The azide ion's utility as a cyclooctyne quenching reagent is demonstrated by using it to spatially pattern uniformly doped hydrogels. The facile quenching of cyclooctynes demonstrated here should be useful in other bioorthogonal ligation techniques in which cyclooctynes are employed, including SPANC, Diels-Alder, and thiol-yne.

Self-Reporting Fluorescent Step-Growth RAFT Polymers Based on Nitrile Imine-Mediated Tetrazole-ene Cycloaddition Chemistry

We introduce an inherently fluorescent self-reporting step-growth polymer system as well as a fluorescence-based methodology for accessing the kinetics of the underpinning photoinduced nitrile imine-mediated tetrazole-ene cycloaddition (NITEC) process, using an equimolar mixture of a bismaleimide linker and a bifunctional α,ω-tetrazole-chain transfer agent (CTA). Similarly, α,ω-tetrazole-capped polystyrene, prepared via RAFT polymerization, was employed as a photoreactive macromonomer. Upon UV irradiation, the tetrazole moiety readily reacts with activated dialkenes producing the fluorescent pyrazoline-containing polymer. Thus, the fluorescence emission of the step-growth polymers is directly correlated with the number of ligation points in the polymer, forming an ideal self-reporting sensor system. The viability of the fluorescence-based quantification is verified via NMR spectroscopy, evidencing that fluorescence-based polymerization monitoring is a viable avenue in cases where NMR spectroscopy is difficult to conduct.