The ability to manipulate the structure and function of promising nanosystems via energy input and external stimuli is emerging as an attractive paradigm for developing reconfigurable and programmable nanomaterials and multifunctional devices. Light stimulus manifestly represents a preferred external physical and chemical tool for in situ remote command of the functional attributes of nanomaterials and nanosystems due to its unique advantages of high spatial and temporal resolution and digital controllability. Photochromic moieties are known to undergo reversible photochemical transformations between different states with distinct properties, which have been extensively introduced into various functional nanosystems such as nanomachines, nanoparticles, nanoelectronics, supramolecular nanoassemblies, and biological nanosystems. The integration of photochromism into these nanosystems has endowed the resultant nanostructures or advanced materials with intriguing photoresponsive behaviors and more sophisticated functions. In this Review, we provide an account of the recent advancements in reversible photocontrol of the structures and functions of photochromic nanosystems and their applications. The important design concepts of such truly advanced materials are discussed, their fabrication methods are emphasized, and their applications are highlighted. The Review is concluded by briefly outlining the challenges that need to be addressed and the opportunities that can be tapped into. We hope that the review of the flourishing and vibrant topic with myriad possibilities would shine light on exploring the future nanoworld by encouraging and opening the windows to meaningful multidisciplinary cooperation of engineers from different backgrounds and scientists from the fields such as chemistry, physics, engineering, biology, nanotechnology and materials science.

Photochromism into nanosystems: towards lighting up the future nanoworld.

This article presents a comprehensive review of recent progress of research dedicated to low-dimensional nanomaterials constructed from functional low-molecular-weight organic compounds, whose optoelectronic properties are fundamentally different from those of their inorganic counterparts. After introducing the development of inorganic and organic macromolecular nanomaterials, we begin with a general review of the construction strategies for achieving both zero-dimensional (OD) and one-dimensional (ID) nanostructures from small organic functional molecules. We then provide an overview of the unique optoelectronic properties induced by molecular aggregation in the nanostructures. Special emphasis is put on the luminescent properties that are different from those of the corresponding bulk materials, such as aggregation-induced enhanced emission, fluorescence narrowing, multi-color emission, and tunable and switchable emissions from doped nanostructures. We conclude with a summary and our personal view of the direction of future development of organic opto-functional nanomaterials and devices.

Low-Dimensional Nanomaterials Based on Small Organic Molecules: Preparation and Optoelectronic Properties†

This tutorial review illustrates the structural design, photochemical and photophysical properties of nanostructured constructs incorporating luminescent and photochromic components. In these systems, the pronounced structural and electronic modifications that accompany the transformations of the photochromic components can be exploited to modulate the emission intensity of the luminescent components on the basis of electron and energy transfer processes. These photoresponsive systems can be assembled by: (1) integrating fluorescent and photochromic components within the main chain of the same polymer; (2) attaching multiple photochromes to a fluorescent organic polymer or luminescent inorganic nanoparticle; (3) appending either independent fluorophores and photochromes or fluorophore–photochrome dyads to a common polymer scaffold; (4) trapping distinct fluorophores and photochromes within the hydrophobic interior of the same cross-linked polymer. In all instances, the changes in absorbance and/or redox potentials associated with the reversible interconversion of the two states of each photochromic component regulate the radiative deactivation of the luminescent components. As a result, the emission intensity of these nanoscaled assemblies can reversibly be switched between high and low values under the influence of optical stimulations. Thus, these clever operating principles for fluorescence modulation can lead to the development of innovative functional and nanostructured materials with photoresponsive character. In particular, protocols for the optical writing and reading of data as well as luminescent probes for bioimaging applications might ultimately emerge from these fundamental studies on photoresponsive molecular switches.

Fluorescence modulation with photochromic switches in nanostructured constructs

Despite considerable efforts toward the development of various sophisticated spiropyrans for metal ion sensing, less attention has been paid to organic molecule sensing One of the major difficulties for detection of organic molecules using a spiropyran is the weak and nonspecific interaction between the spiropyran and the target Here, we report the synthesis and molecular recognition characterization of two bis-spiropyrans for dipolar molecules and their application to in vivo glutathione (GSH) fluorescent probes Unlike the mono-spiropyrans, the newly designed bis-spiropyran molecules feature a rigidly maintained molecular cleft and two spiropyran units as binding modules The molecular recognition is based on multipoint electrostatic interactions and structure complementarity between the opened merocyanine form of the spiropyran and the analyte It was observed that the spiropyran 1a binds GSH in aqueous solution with high affinity (K = (752 +/- 183) x 10(4) M(-1)) and shows strong fluorescence emission upon binding Remarkably, fluorescence output of 1a is not significantly affected by other amino acids and peptides, especially, structurally similar compounds, such as cysteine and homocysteine Furthermore, fluorescence anisotropy and confocal fluorescent microscopy confirmed that spiropyran 1a is a comparatively good candidate for intracellular delivery and can be accumulated intensively into cells Thus, 1a can be utilized in vivo as a GSH probe or as a marker to show the level of intracellular GSH

Design of bis-spiropyran ligands as dipolar molecule receptors and application to in vivo glutathione fluorescent probes

Synthesis of para-sulfonatocalix[4]arene-modified silver nanoparticles as colorimetric histidine probes.

Basic amino acids (BAA) induced 1-(β-carboxyethyl)-3,3-dimethyl-6′-nitrospiro(indoline-2,2′[2H-1]benzopyran) (SP) isomerization to its open form, i.e.merocyanine (MC), and the formation of its complex with BAA in water was reported for the first time, which may provide a potential application for in situ recognition of BAA.

Basic amino acid induced isomerization of a spiropyran: towards visual recognition of basic amino acids in water

We describe a versatile and convenient approach to achieve fluorescence modulation by the preparation of composite nanoparticles (CNPs), based on photochromic 5-methoxy-1,3,3-trimethyl-9'-hydroxyspiroindolinenaphthoxazine (SO), fluorescent 4-(dicyanomethylene)-2-methyl-6-(p-dimethyl-aminostyryl)-4H-pyran (DCM), and emissive-assistant 1,3-bis(pyrene) propane (BPP) molecules, employing doping techniques. The mechanism of the fluorescence switch is the intermolecular energy transfer as supported by both steady-state and time-resolved spectroscopy results. The addition of BPP not only enhances the contrast of the fluorescence signal between the 'ON' and 'OFF' state, but also provides a convenient way to tune the excitation wavelength for reading the fluorescence. High-contrast ON/OFF (20:1) fluorescence switching is successfully implemented in the CNPs and also in a more practical PVA film loaded with the CNPs. This system may represent an alternative to the covalent system in potentially rewritable high-density optical data or image storage utilizing luminescence intensity readout schemes.

http://iopscience.iop.org/article/10.1088/0957-4484/18/14/145707/pdf

Modulation of a fluorescence switch based on photochromic spirooxazine in composite organic nanoparticles

Self-assembly of organic molecules from solution is one of the simplest methods to generate ordered nanostructures with potentially new properties. In particular, nanostructured architectures on the macroscopic scale have possible applications in the fields of electronics, catalysis, and medicine. However, controllable fabrication of nanostructured materials is still limited by the available processing methods. Template synthesis has been widely used as a controllable approach to achieve desirable nanostructured materials. Of the many different types of templates, anodic aluminum oxide (AAO) offers clear advantages in the making of onedimensional nanostructured materials and arrays; the AAO templates provide hexagonally packed, uniform pore arrays with a pore diameter that can be varied up to 200 nm. Amongst other applications, AAO has been used as a template for the syntheses of nanotubes for biomedicine and biotechnology, Bi1 xSbx nanowires as thermoelectric wires, SBA-15 nanorod arrays for protein separation and catalysis, and lipid nanotube arrays as a model of cellular membranes. Despite such progress, there have been few reports on the application of AAO as a substrate for the control of surface morphology on the macroscopic scale. Herein, we report for the first time the synthesis of the largearea (ca. 12 cm) nanonet architecture of 5,10,15,20-tetrakis(p-chlorophenyl)porphyrin (TClPP; C44H26Cl4N4) using the AAO template as a substrate. Figure 1 shows the 3D structure of the TClPP molecule and its stacking. Alkylated polycyclic discotic molecules, such as porphyrins, are frequently employed as building blocks because of their ability to stack and form architectures and liquidcrystalline phases. We were able to use the stacking property of TClPP to successfully cultivate nanonet network architectures on the AAO templates. The TClPP nanonets were fabricated as follows. A 2-cm-diameter AAO disk about 15 mm in thickness was laid on a Buchner funnel fitted with a fritted disk. The funnel was then placed on a filter flask connected to a vacuum pump, which was used to maintain a pressure differential across the AAO disk. A solution of TClPP (1 mL, 0.13m) in CH2Cl2 was added dropwise to the AAO template. TClPP nanonets of different meshes in accordance with the AAO pore sizes were thus grown on the rear of the AAO template, that is, the side opposite to that of TClPP deposition. This process was repeated several times to obtain nanoparticles of larger size. Experimental parameters, such as the pressure differential, concentration of the TClPP solution, and the number of times of solution deposition, affected the type and the quality of the nanostructures formed, which will be discussed below. The morphology and size of the nanostructures were examined by field-emission scanning electron microscopy (SEM; Hitachi S-4300). Figures 2a and b show the SEM images of the TClPP nanonet structures formed on the AAO templates. It is evident that the nanonet structures were created as a result of uniform self-assembly of interlocking TClPP nanoparticles. The SEM images also suggest that the knots of the nanonets were formed by a single larger nanoparticle or by several assembled nanoparticles. In Figure 2a, the inner pores of the AAO substrate can be seen behind the floating nanonet. An advantage of this synthesis method was that the fabricated nanonet structure could be easily removed from the AAO substrate. Figure 2b shows the SEM image of a nanonet after it was removed from the substrate. The weblike nanonets showed ordered network structures with meshes mimicking the pore sizes of the AAO templates. Apparently the orderly pores of the AAO template aided the formation of the periodic pattern of the nanonets on the back surface of the template. Three different pore sizes (50, 100, and 200 nm) of the AAO templates were tried in the making of nanonets. A small Figure 1. 3D structure of the TClPP molecule (left) and its stacking (right).

Controllable nanonet assembly utilizing a pressure-difference method based on anodic aluminum oxide templates.

A fluorescence switch by the photoisomerization of a photochromic compound in CH3CN and in a polymer film using a bistable photochromic (1,2-bis(2-methylbenzo[b]thiophen-3-yl) hexafluorocyclopentene) (BTF6) and a fluorescent 3-(2-benzothiazolyl)-7-(diethylamino) coumarin (coumarin6) was demonstrated. Because only the closed form of BTF6 serves as a fluorescence quencher of coumarin6, and the read (406 nm), write (254 nm), and erase (>500 nm) wavelengths are well-separated, a reversible modulation of the fluorescence of coumarin6 with high contrast and high sensitivity is expected to be realized. This system may represent an alternative to the fluorescence switches that are based on covalent systems in the potentially long-term optical data or image storage schemes utilizing luminescence intensity readout.

Yuanyuan Liu

Papers

Basic amino acid induced isomerization of a spiropyran: towards visual recognition of basic amino acids in water

Modulation of a fluorescence switch based on photochromic spirooxazine in composite organic nanoparticles

Controllable nanonet assembly utilizing a pressure-difference method based on anodic aluminum oxide templates.

Reversible fluorescence modulation based on photochromic diarylethene and fluorescent coumarin

Photochromism induced aggregate-monomer interconversion and fluorescence switch of porphyrin with spiropyran