Photochromic nanoparticles

Elizabeth J. Harbrona

DOI: 10.1039/9781849732826-00211

Nanoparticles have been doped or functionalized with photochromicmolecules for a variety of applications that take advantage of the photo-induced changes in molecular properties that accompany a photochromictransformation. Many photochromic nanoparticles are designed to befluorescent and to undergo reversible changes in fluorescence intensity orcolor upon irradiation. Such systems rely on the photochromic transfor-mation to generate a fluorescence quencher. Other photochromic nano-particle systems harness photoinduced changes in other molecularproperties such as polarity and molecular size. This review features pho-tochromic nanoparticles from the recent literature and gives examples ofboth organic and inorganic nanoparticles that have been doped or cova-lently functionalized with organic photochromic molecules.

1 Introduction

Photochromic molecules undergo conversion from one molecular form toanother in response to specific wavelengths of light.1–4 By definition, the twomolecular forms of a photochromic molecule have different light absorptionproperties, i.e., color. This change in absorbance upon photochromic con-version is accompanied by changes in a host of other molecular propertiesincluding shape, size, dipole moment, and redox potentials. The photo-induced change of any one of these properties can be exploited as the basisfor light-induced applications.

Scheme 1 shows the structures of photochromes from four of the mostcommonly employed photochromic families, azobenzenes (1), spiropyrans(2), spirooxazines (3), and dithienylethenes (4). In all four of these examples,the initial form of the molecule (1a-4a) absorbs light exclusively in theultraviolet (UV) region of the spectrum. Upon UV irradiation, theyundergo conversion to their visible-absorbing forms (1b-4b), which may bethermally stable (4b) or revert back to their initial form over time (1b-3b).The selection of a thermally bistable photochrome such as 4 or one thatspontaneously converts from its photogenerated form back to its initialform (1-3) is generally dictated by the nature of the application.

The incorporation of small molecule photochromes such as 1-4 intonanoparticle systems has become a topic of intense interest due to thepowerful combination of photomodulation with particle properties.5–7

Nanoparticles may be composed entirely of small molecules, organicpolymers doped or functionalized with small molecules, or metals that aresurface-functionalized with small molecules. These very different nano-particle systems share in common the fact that a large number of photo-chromes can be incorporated into a small area within them and/or on their

aDepartment of Chemistry, College of William and Mary, P.O. Box 8795, Williamsburg, VA,USA

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�c The Royal Society of Chemistry 2011

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surfaces. Thus, a much larger response to light can be generated in thesenanoparticles than in small molecules dispersed in a typical solution orsolid sample. In many cases, incorporation of the photochromes within ananoparticle structure also protects them from light-induced degradationprocesses, known as fatigue, that limit the number of photocycles a photo-hrome can undergo in typical samples.8

Additionally, many nanoparticles are designed to be suspended inaqueous solution, and incorporation of photochromes into these particlesoffers a means of getting these often-hydrophobic small molecules intowater. This aspect of photochromic nanoparticles is particularly importantfor the growing number of applications of photochromism in biologicalimaging using conventional or ultrahigh resolution microscopy methods.9

This review highlights some of the most recent work in the area ofphotochromic nanoparticles. It focuses on nanoparticles that incorporatean organic photochrome and demonstrate the reversible photoswitching ofone or more properties. It is not intended to be historical or comprehensivein nature but rather to highlight some of the most exciting examples of

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Scheme 1 Examples from common photochromic families.

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photochromic nanoparticles in the recent literature. Another recent reviewfocuses specifically on photochromic nanoparticles for fluorescence mod-ulation applications.5

2 All-organic photochromic nanoparticles

2.1 Overview

Photochromic nanoparticles composed entirely of organic materials maycontain only small molecules or may combine small molecule photochromeswith organic polymers. The vast majority of all-organic photochromicnanoparticles have been designed for fluorescence modulation. The mostcommon mechanism by which fluorescence modulation is achieved in thesesystems is depicted in Scheme 2, which illustrates a fluorophore (FL)associated with a photochrome in its initial form (PC-A).

The fluorophore and photochrome functionalities may be incorporatedinto the same molecule or, more commonly, may be separate moleculesfixed in close proximity by covalent bonds or the nanoparticle matrix. Whenthe photochrome is in its initial UV-absorbing form, it has no interactionwith the fluorophore, which emits upon excitation as it would in the absenceof the photochrome. UV light initiates the photochromic conversionthrough which the visible-absorbing alternate form of the photochrome isformed (PC-B). While PC-A had no interaction with the fluorophore, PC-Bacts as an energy transfer acceptor for the fluorophore via a fluorescenceresonance energy transfer (FRET) mechanism. Thus, PC-B efficientlyquenches the fluorescence of the fluorophore, which will not emit until thereverse photochromic conversion back to PC-A occurs.

There are many possible variations on this scenario, including the use ofphotoinduced electron transfer (PET) rather than FRET as the quenchingmechanism, but the core feature of light-induced fluorescence modulationremains the same. Further variations and details are included in reviews offluorescence photomodulation.6,10 Applications of fluorescence modulationinclude optical data storage2 and probes for ultrahigh resolutionmicroscopy.9

2.2 Nanoparticles from small molecules

Organic nanoparticles composed only of small molecules can potentiallycontain the highest density of photochromes among the different types of

Scheme 2 Fluorescence modulation schematic.

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photochromic nanoparticles described herein. In their simplest form, thesenanoparticles contain only the photochromic molecule as the sole compo-nent. Park and co-workers11 are among many12–18 who have used repreci-pitation to produce such single-component photochromic nanoparticles. Inthis method the photochrome is dissolved in a water-miscible solvent suchas THF, and a small portion of this solution is injected into a much largervolume of water while stirring. Nanoparticle size can be controlled by theconcentration of the initial photochrome solution and ranges from 40 nmup to the hundreds of nm.11 While the reprecipitation method is the mostcommon, single-component nanoparticles can also be prepared by othermethods, including ablation,19,20 vapor deposition and annealing,21 and sol-gel techniques.22,23

Sheng, et al. adapted single-component nanoparticle preparation meth-ods to create nanoparticles containing 2 or 3 types of small molecules forfluorescence modulation applications.15,16 Binary nanoparticles, 40-50 nmin diameter, were prepared with the photochromic dithienylethene (DTE)derivative 4 (Scheme 1) and the fluorescent dye DHBIA.16 DHBIA displaysaggregation-induced emission enhancement, exhibiting weak fluorescence inTHF solution (Ffl,THF=0.0017)24 and much stronger fluorescence when inan aggregated state, such as in nanoparticles composed solely of DHBIA(Ffl,NP=0.205)16. Its fluorescence spectrum (lmax,fl=520 nm) has nooverlap with the UV-absorbing 4a but strong overlap with 4b’s visibleabsorbance band (lmax,abs=522 nm).

Thus, the photochromic conversion should induce a change from brightfluorescence to fluorescence strongly quenched by FRET. As expected, anaqueous suspension of the DTE-DHBIA nanoparticles exhibits moderatelyintense fluorescence when the DTE is in its open 4a form (Ffl,NP=0.076).Upon UV irradiation to induce the 4a-4b conversion, the fluorescenceintensity drops by nearly an order of magnitude (Ffl,NP=0.0073). Thefluorescence modulation is reproducible over multiple cycles of UV andvisible irradiation, highlighting the well-known stability of the DTE pho-tochrome as well as that of the fluorophore. The same reproducible mod-ulation is also observed when the nanoparticles are doped into a poly(vinylalcohol) film.

It is noteworthy that the fluorescence quantum yield of DHBIA in thebinary nanoparticles prior to irradiation is significantly lower than in thecontrol nanoparticles (Ffl,NP=0.076 vs. 0.205). Strong aggregation isrequired for intense DHBIA fluorescence, and the authors note that DTE 4

may decrease the aggregation of DHBIA in the nanoparticles. Indeed, thefluorescence quantum yield improves to 0.150 when the molar ratio of 4 toDHBIA is decreased from 10:1 to 4:1.

2.3 Nanoparticles from organic polymers

Not all photochromes are able to form uniform, stable nanoparticles withno other molecular components involved. For example, Sun, et al.attempted to prepare nanoparticles from various DTE derivatives usinga typical reprecipitation method and found that DTEs with phenyl andp-methoxyphenyl substituents formed irregular crystal grains rather thanregular nanoparticles.12 The same method yielded uniform, stable

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nanoparticles when a DTE derivative with a p-hydroxyphenyl was used.The difference was attributed to the polar hydroxyl group’s hydrogenbonding capacity as well as its ability to induce a negative zeta potential inthe particles that stabilizes them in aqueous suspension.

The incorporation of organic polymers into photochromic nanoparticlescan eliminate the reliance of successful nanoparticle formation on a specificstructure or substituent. Nanoparticles with polymeric components can bedesigned to take advantage of specific polymer properties such as amphi-philicity. Polymers in nanoparticles can also encapsulate photochromiccomponents in a fashion that protects the small molecule photochromesfrom degradation in many cases. For these and other reasons detailedbelow, many photochromic nanoparticles contain organic polymers inaddition to small molecule photochromes.

Photochromic nanoparticles with polymeric components can be createdvia methods in which polymerization and nanoparticle formation are one inthe same process or methods in which polymers are synthesized andnanoparticles are subsequently fabricated from them in a separate proce-dure. Microemulsion polymerization techniques are typically used in theformer approach while the latter includes reprecipitation and self-assemblymethods.5

Li and coworkers have developed a family of polymeric nanoparticleswith spiropyran photochromes for fluorescence modulation applications.To create the nanoparticles, a spiropyran-containing acrylate derivative isco-polymerized with N-isopropylacrylamide (NIPAM) and styrene, withdivinyl benzene as a crosslinker in a microemulsion polymerization.8 Theresulting nanoparticles possess a hydrophilic NIPAM shell that confersstability on aqueous suspensions of particles and a hydrophobic, cross-linked core that contains the photochromic spiropyran units. Nanoparticlesize is controlled through the feed ratios of monomers and surfactants andcan be varied from 40 to 400 nm.8

The spiropyrans encapsulated in these polymeric nanoparticles demon-strate both expected and novel properties. The UV-induced conversion ofspiropyran (SP, 2a) to merocyanine (MC, 2b) occurs with a quantum yield of42% in methanol solution and 28% in the nanoparticles.8 This decrease inquantum yield reflects the difference in free volume available for the photo-chromic conversion in dilute solution versus the polymeric nanoparticle andis typical for photochromes in constrained media.25 Remarkably, the nano-particles exhibit strong fluorescence (Ffl=0.24) when the photochrome is inits merocyanine form (2b). Merocyanines are weakly fluorescent at best insolution but emit strongly here due to their rigid, hydrophobic environmentthat restricts nonradiative decay pathways. The nanoparticle environmentalso protects the photochrome from photodegradation processes that tendto be particularly deleterious for spiropyran-derived photochromes. Nano-particles with encapsulated spiropyrans have been further developed by theLi group26,27 and others28–32 for fluorescence modulation.

Recent work by Li demonstrates how the spiropyran nanoparticles canbe used to overcome two of the major drawbacks of photochromic nano-particles in biological imaging: sample damage caused by the UV irradia-tion required to induce most photochromic conversions and unwanted

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photochromic reaction that occurs when the light needed to excite afluorophore also induces photochromic conversion.33 UV light inducesSP-MC conversion and can excite MC fluorescence but also bleaches theMC fluorophore over time and causes increased background and photo-damage in biological samples. Visible light can excite MC fluorescence butalso induces the MC-SP back-conversion unless the wavelength is verycarefully chosen. For sustained fluorescence, light that excites the MCfluorophore without inducing photoswitching or bleaching is required.

Two-photon techniques solve these problems: simultaneous absorption oftwo near-IR (NIR) photons not only induces SP-MC conversion but alsoexcites MC, switching on red fluorescence.33 Furthermore, the red fluores-cence grows more intense over time as additional photochromes are switchedto their fluorescent MC form. Fig. 1 illustrates this phenomenon for two-photon NIR absorption of spiropyran nanoparticles on the surface of humanbreast cancer cells. The nanoparticles were functionalized with antibodiesthat bind to a cancer marker that is overexpressed on the surface of breastcancer cells. The sustained fluorescence observed in these images is a strongcontrast to one-photon irradiation, which causes fluorescence intensity todecrease with time due to bleaching (lexc=UV) or photochromic back-conversion (lexc=visible). The nanoparticle fluorescence can be switched offwith one photon of visible light to induce MC-SP back-conversion, and thesystem is stable to repeated two-photon-on/one-photon-off cycles.

In addition to the conventional images such as those in Fig. 1, the SPnanoparticles can also be used to obtain ultrahigh resolution images withresolution beyond the diffraction limit of light. Hu and Li have demon-strated application of the nanoparticles to photoactuated unimolecularlogical switching attained reconstruction (PULSAR) microscopy, which canproduce images with resolution in the range of 10-40 nm.34 This techniquerelies on temporal switching of fluorophores to achieve its high resolution:the position of a fluorophore can be determined with high precisionprovided that any neighboring fluorophores are switched to an off or darkstate at the time of imaging.

For SP nanoparticle samples, a short, low power UV laser pulse inducesonly a few photochromes to convert to MC and become fluorescent.The fluorophores are imaged with 532 nm light and are subsequently

Fig. 1 Time dependence of two-photon photoswitching and two-photon imaging of anti-Her2antibody-conjugated photoswitchable nanoparticles anchored on the Her2 receptors of SK-BR-3 cells. Continuous illumination using the NIR laser at 780 nm steadily induced SP-to-MCisomerization and hence intensified the red fluorescence signal. Reprinted with permission fromref. 33. Copyright 2011, American Chemical Society.

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photobleached so that their fluorescence cannot interfere with the locali-zation of other nearby fluorophores that will be switched on in future cycles.Repeated UV/532-nm irradiation cycles enable the precise localization of ahigh number of individual fluorophores via fitting of each image peak by aGaussian mask to determine its centroid position.

Fig. 2 compares the resolution of this method to conventional imaging for70 nm SP nanoparticles. The differences in the two sets of images are dra-matic: the resolution in the PULSAR images is B25-fold better than in theconventional images, and nanoscale features that are completely obscuredin the conventional images are resolved. PULSAR imaging using SPnanoparticles can also be applied to biological systems.34

The same PULSAR technique that was applied to the imaging of prox-imate particles above can also be used to learn more about the nanoparticlesthemselves.35 The spiropyran photochromes are presumed to be located inthe hydrophobic core of the nanoparticles based on indirect and ensemble-averaged data. The strong MC fluorescence indicates a constrained andhydrophobic environment as MC does not fluoresce appreciably in solutionor when attached to polymers that do not form micelles or other con-strained structures.8

MC is also strongly solvatochromic, and the position of its absorptionband within the nanoparticles (588 nm) suggests a nonpolar environmentwhen compared to its absorbance in water (525 nm) and toluene (600 nm).

Fig. 3 shows three different pairs of SP nanoparticles in close proximity toeach other.35 In 3a and 3c, the nanoparticles are in contact while 3b showstwo nanoparticles that are slightly apart from each other.

As shown in scatterplots in Fig. 3d-f, the PULSAR localization of thefluorescence of individual MC emitters reveals their distribution in thenanoparticles. Even though the nanoparticles in 3a and 3c are in contact,their emitting dyes are not in close proximity and are clustered into twoseparate groups at the center of each particle. This result strongly supportsthe previous conclusion that the photochromes reside in the hydrophobiccore of the nanoparticles. Particles with photochromes in the hydrophilicshells would have appeared as hollow rings in the PULSAR images.

Fig. 2 Using 70-nm nanoparticles to compare conventional fluorescent microscopy (top) andPULSAR microscopy (bottom) reveals the resolution difference in the nanoparticle patterns.Reprinted with permission from ref. 34. Copyright 2008, American Chemical Society.

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The SP nanoparticles described above were all prepared by microemul-sion polymerization, in which the covalent connection of monomer unitsand the creation of the spherical nanoparticle structure occur as part of thesame process. An alternative approach is to create a polymer backbone andthen use a separate process to fabricate the spherical nanoparticle geometry.In the latter method, the photochrome may be covalently attached to alinear polymer backbone or may be non-covalently doped into the nano-particle during its preparation. Several examples below highlight differentstrategies for formation of nanoparticles from existing polymer chains.

Raymo and coworkers used a doping technique in the creation ofspiropyran nanoparticles for fluorescence modulation.36 While separatepolymerization and particle preparation steps are required in this method,the additional synthetic steps required to functionalize photochromes andother components for covalent incorporation in the polymer structure areavoided. The nanoparticles contain three components: amphiphilic polymer5 (Scheme 3), a photochromic spiropyran related to 2, and the fluorophoreBODIPY (6). The spiropyran-BODIPY combination was selected for itsfluorescence modulation ability. The spiropyran in its closed form has nointeraction with the BODIPY fluorophore but has the requisite spectraloverlap to act as an energy transfer acceptor in its open merocyanine form.

Furthermore, electron transfer from BODIPY to the photochrome isendergonic for the SP form but exergonic for the MC form, making electrontransfer a second potential fluorescence quenching mechanism for thissystem. Both the photochrome and fluorophore are functionalized withhydrophobic decyl chains to ensure their encapsulation in the nanoparticle

(a) (b) (c)

(d) (e) (f)

Fig. 3 Selected nanoparticle pairs show close or nearly close contact at nanometre resolutionand their images are not complicated by the presence of other nanoparticles in the vicinity. Theimages in (a), (b), and (c) are presented as scattering plots in (d), (e), and (f), where each dotrepresents a fluorescence event from MC dyes. Reprinted with permission from ref. 35.Copyright 2011, Royal Society of Chemistry.

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rather than in the aqueous phase. The nanoparticles in this system areessentially polymer micelles formed when the amphiphilic copolymer isdispersed in water. Undoped polymer micelles are formed from an averageof four copolymer chains and have an average diameter of 18 nm (hydro-dynamic diameter, dynamic light scattering) or 17 nm (TEM).

When prepared with the spiropyran and BODIPY dopants, expandedmicelles composed of 19 copolymer chains form and have an average dia-meter of 37 nm (hydrodynamic radius) or 35 nm (TEM). Experimentalevidence supports the expectation that the hydrophobic dyes will localize inthe hydrophobic interior of the polymer micelle. The solvent-sensitiveabsorption spectrum of the photogenerated merocyanine is consistent witha hydrophobic environment. Additionally, the BODIPY fluorophoreundergoes substantial photobleaching in aqueous environments that appearsto be reduced in the micelle, arguing for the fluorophore’s presence in theprotected interior of the micelle.

The SP- and BODIPY-doped particles exhibit fluorescence modulation,as expected.36 BODIPY fluorescence is observed when the photochromesare in their non-quenching SP form. The BODIPY fluorescence intensitydrops to ca. 20% of its initial value upon UV-induced SP-MC conversion.A small amount of red fluorescence from MC is observed, consistent withthe hydrophobic environment of the photochrome. The fluorescencerecovers thermally in the dark or upon application of visible light. Repeatedcycles exhibit some loss of fluorescence intensity over time, which variousexperiments demonstrated was due to photodegradation of the BODIPYfluorophore more so than the photochrome. In addition to demonstratingfluorescence modulation in an aqueous buffer suspension, Raymo andcoworkers also showed that the particles could be used in cellular imaging.Chinese hamster ovarian (CHO) cells were incubated with a buffered sus-pension of the SP- and BODIPY-doped polymer micelles.

Fig. 4 shows the CHO cells before (A) and after (B) incubation. Fluor-escence is observed only after incubation, demonstrating that the polymermicelles cross the cell membrane. The micelles appear to localize pre-ferentially in the cytosol with some accumulation in the nucleus.

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Scheme 3 Amphiphilic polymer 5 and BODIPY fluorophore 6 from ref. 36.

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Application of UV light for 30 s caused the fluorescence intensity of themicelles in CHO cells to decrease to ca. 40% of the initial value, andrecovery is observed upon storage of the sample in ambient light (notshown). Cell viability studies were also conducted with the conclusion thatneither the presence of the particles nor the UV irradiation were cytotoxic.

Harbron et al. also used a doping method to prepare polymer nanoparticlesfor fluorescence modulation.37 The unique feature of these nanoparticles isthat the polymer itself is the fluorophore in this fluorophore-photochromesystem. The fluorescent conjugated polymer MEH-PPV acts as the FRETdonor while a spirooxazine (3) has no interaction with the polymer in itsclosed form (SO, 3a) but is an efficient FRET acceptor in its open mer-ocyanine form (MC, 3b) due to the spectral overlap of the MEH-PPVfluorescence with the MC absorbance. Although closely related to thespiropyran (2) used in the polymer nanoparticles of Li and Raymo descri-bed above, the spirooxazine is known to have increased photostability andresistance to fatigue compared to the spiropyran.38

The SO-doped MEH-PPV nanoparticles were prepared by a surfactant-free reprecipitation process optimized for conjugated polymers by McNeilland co-workers.39 Briefly, MEH-PPV is dissolved in THF and combined witha solution of SO in THF to obtain the desired photochrome-to-polymer ratio.A small portion of this dilute precursor solution is injected rapidly into water,where the hydrophobic SO dye partitions with MEH-PPV to form dopednanoparticles with an average size of 8 nm that are stable in aqueous sus-pension. Kee and coworkers have demonstrated that the stability of thesesurfactant-free nanoparticles is due to a negatively charged surface formed byoxygen-containing hydrophilic chemical defects on the particle surface.40

Fig. 5 shows the fluorescence photomodulation obtained with the SO-doped MEH-PPV nanoparticles. In its thermally stable SO form (3a), thephotochrome has no impact on the polymer fluorescence: spectral position,shape, and brightness are indistinguishable from undoped control nano-particles. Upon UV irradiation to induce SO-MC conversion, the

(a) (b)

Fig. 4 Confocal fluorescence images (lex=514 nm, lem=535 – 635 nm, scale bar 50 mm) ofCHO cells recorded before (a) and after (b) incubation with a PBS dispersion (10%, v/v) ofspiropyran- and BODIPY-doped polymer micelles for 24 h. Reprinted with permission fromref. 36. Copyright 2011, American Chemical Society.

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fluorescence is quenched to 7.5% of its initial intensity. The fluorescenceintensity recovers fully upon thermal MC-SO conversion and can becycled upon repeated application of UV irradiation and dark thermalrecovery periods (Fig. 5B).

As expected, the extent of fluorescence quenching depended on the con-centration of MC FRET acceptors generated in each sample. Analysis ofquenching versus concentration data indicated that some particles were likelycompletely quenched while others were only partially quenched and wereresponsible for the residual emission. Indeed, McNeill and co-workers pre-pared similar particles using an SOdye and a different conjugated polymer andobserved complete quenching of individual particles, as shown in Fig. 6.41

The examples of polymer-based photochromic nanoparticles describedabove were all developed for fluorescence modulation applications. In con-trast, Branda and co-workers created novel DTE-containing polymer nano-particles that exhibit reversible changes in size in response to two differentstimuli, light and temperature.42 Hydrophilic poly(N-isopropylacrylamide),

(A) (B)

Fig. 5 (A) Fluorescence spectra of 28 wt % SO-doped MEH-PPV nanoparticles suspended inwater before (solid) and after UV irradiation (dotted) and after thermal recovery (dashed). (B)Fluorescence intensity of the doped nanoparticles at 590 nm after UV irradiation (low-intensitypoints) and thermal recovery (high-intensity points) over many cycles. Reprinted with per-mission from ref. 37. Copyright 2009, American Chemical Society.

Fig. 6 Single particle photoswitching of SO-doped poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1’,3}-thiadiazole)] nanoparticles. A 405 nm diode laser is used both to induce theswitching to the ‘‘OFF’’ state and to induce fluorescence. The fluorescence from single particleswas rapidly switched off in a few tens of ms under continuous laser illumination. The ‘‘ON’’state recovers after turning off the laser for a few s at room temperature. Reprinted withpermission from ref. 41. Copyright 2010, Royal Society of Chemistry.

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a component of Li’s SP nanoparticles above, was used by Branda as thebackbone of an amphiphilic, photochromic copolymer. A hydrophobic DTE-functionalized acrylamide monomer was synthesized and copolymerized withN-isopropylacrylamide to produce the amphiphilic copolymer. A modifiedreprecipitation procedure was used to prepare the nanoparticles from thisDTE-labeled copolymer. A large amount of water was added to a solution ofthe copolymer inTHF, and the THFwas then removed by rotary evaporation.

The nanoparticle preparation was carried out under UV irradiation toproduce particles in which the DTEs were in their visible-absorbing closedforms and under visible light to produce particles with open-form DTEs. Theparticles produced by this method were uniform, had an average diameter of150 nm, and were extremely stable in aqueous suspension. Analysis ofabsorption data revealed that a remarkable 98% of DTEs within the nano-particles photoconverted between their ring-open and ring-closed forms. Thenanoparticle geometry typically imposes free volume constraints that reducephotochromic conversions inmost systems, but the high conversions observedhere indicate that this is not the case for this system.

The efficient photochromic conversion observed in these amphiphilicnanoparticles affects not only the absorption spectra but also the physicaldimensions of the nanoparticles. When the DTEs are in their visible-absorbing closed form, the particles occupy a larger volume than when theDTEs are in their UV-absorbing open form, as shown in Fig. 7A. Althoughthe difference in size is small, on the order of a few nanometers’ change inhydrodynamic diameter, it is highly reproducible (Fig. 7B).

The DTE closed form is known to have a more rigid structure than theopen form, and the change in shape and rigidity of the pendant DTEs uponring closure may be responsible for the increase in nanoparticle diameterthat accompanies the photochromic conversion. The hydrodynamic dia-meters of the nanoparticles can also be modulated by temperature, with theparticles becoming smaller as the temperature is increased. The change inparticle diameter with temperature is attributed to changes in the extent ofswelling of water molecules into the nanoparticles. As the temperature

(A) (B)

Fig. 7 (A) Average hydrodynamic diameter changes of nanoparticles with photochromes inthe open form (P1-o) and with the photostationary state concentration of closed photochromes(P1-pss) at various temperatures. (D) Reversible switching of average hydrodynamic diamtersof P1-o nanoparticles with alternate exposure to UV light (313 nm, 5.5 mw cm� 2, 2 min; dashedline) and visible light (W500 nm, 300 W, 5 min; solid line) at 20 1C and with alternating high(60 1C; solid line) and low (20 1C; dashed line) temperatures at the 313 nm photostationarystate. Reprinted with permission from ref. 42. Copyright 2011, Elsevier.

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increases, hydrogen bonding between polymer subunits becomes morefavorable than that between water and polymer units. With fewer watermolecules hydrogen-bonded in the nanoparticles at higher temperatures, theparticles become smaller. At every temperature studied (10–60 1C, Fig. 7A),the particle size can be modulated by light with the closed-form particlesbeing consistently larger. The combination of the DTE structural changeand swelling due to hydrogen-bonded water molecules likely govern thereversible changes in particle size.

3 Inorganic nanoparticles with organic photochromes

3.1 Overview

The vast majority of all-organic photochromic nanoparticles describedabove were developed for fluorescence modulation applications. In con-trast, literature reports of inorganic nanoparticles functionalized withorganic photochromes reflect a much broader diversity of applications.Modulation of magnetic properties has been achieved with iron oxidenanoparticles functionalized with DTEs43 or azobenzenes,44 azobenzene-derivatized zinc oxide nanoparticles,45 and azobenzene-functionalizedPrussian blue nanoparticles.46 Photomodulation of helicity was demon-strated in gold nanoparticles functionalized with a novel photochrome thatacts as a light-driven molecular motor.47 Spiropyran-capped gold nano-particles exhibited photomodulated binding of amino acids.48 Azobenzene-derivatized gold or silver nanoparticles have shown photomodulation ofaggregation with interesting consequences in several cases.49–54

Derivatization of gold or silver nanoparticles with DTEs for the photo-modulation of conductance55–59 or the local surface plasmon resonance60 hasalso been demonstrated. There have been a number of reports of fluorescencemodulation in inorganic nanoparticles, including the outstanding report ofsilica nanoparticles designed for ultrahigh resolution microscopy from Bossiand coworkers.61 Several groups have also functionalized quantum dots withphotochromes for fluorescence modulation.62–67 A few recent examples ofinorganic photochromic nanoparticles will be presented herein.

3.2 Examples

An exemplar of the many non-fluorescent applications of photochromicinorganic nanoparticles comes from Grzybowski and coworkers, who havedeveloped intriguing applications for photomodulated aggregation.49–51 Todevelop disappearing ‘‘inks,’’ they functionalized gold or silver nano-particles with azobenzene-terminated alkanethiols and dispersed them inorganic nanogels.50 When the azobenzenes are in their thermally stable transisomeric form, they are nonpolar and do not aggregate. UV irradiationproduces cis azobenzene isomers, which are much more polar than theirtrans counterparts.

The increased polarity promotes aggregation of the azobenzene-functio-nalized nanoparticles into superspherical assemblies, which exhibit a differentcolor to the naked eye than the individual nanoparticles. Trans-cis azo-benzene isomerization induces a color change from red to pale blue in goldnanoparticles and yellow to violet in silver nanoparticles. Images were

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produced by irradiation of the nanogels through a photomask, and theseimages were gradually erased by azobenzene back-isomerization over time-scales of hours to days when left in ambient light. The samples were stable toat least hundreds of cycles of writing and erasing and were also flexible,increasing the number of possible applications for the images. The sameaggregation phenomenon has also been used to create photoswitchablecatalysts.49

Liu and coworkers developed a novel fluorescence temperature sensorbased on silica nanoparticles grafted with a fluorophore- and photochrome-functionalized thermoresponsive polymer.68 The organic polymer is basedon poly(N-isopropylacrylamide) (PNIPAM), which was also used in thework of Branda described in 2.3 above. The silica nanoparticle surface iscoated with PNIPAM chains via a sequential surface-initiated atom transferradical polymerization that creates an 8-nm-thick layer of polymer chainssurrounding the 45-nm-diameter silica nanoparticles. PNIPAM in this workwas randomly labeled with fluorophore 7-nitro-2,1,3-benzoxadiazole(NBDAE) and a spiropyran photochrome. NBDAE’s fluorescence(lmax,fl=530 nm) overlaps with the absorbance of the photogenerated mer-ocyanine, so quenching of theNBDAEfluorescence by aFRETmechanism isobserved when the photochrome is in its merocyanine form. Merocyaninefluorescence (lmax,fl=618 nm) is observed as a consequence of the FRET.

The NBDAE- and SP-functionalized PNIPAM layer contracts withincreasing temperature over the 20 – 37 1C range, exhibiting a continuousdecrease in the hydrodynamic radius of the PNIPAM-functionalizednanoparticles. Because of the FRET efficiency’s dependence on donor-acceptor distance, the NBDAE-MC FRET efficiency changes as thedimensions of the PNIPAM layer change with temperature. As the tem-perature increases, the PNIPAM contracts, bringing NBDAE donors andphotochromic acceptors closer together and increasing the number ofdonor-acceptor pairs that are close enough to undergo FRET. Thus, thefluorescence intensity of NBDAE decreases while that of MC increases as

(a) (b)

Fig. 8 (A) Fluorescence spectra (lex=480 nm, slit widths: ex 4 nm, em 4 nm) and (B)fluorescence intensity ratio changes, F618/F530, in the temperature range of 20–37 1C obtainedfor the aqueous dispersion (0.25 g/L) of hybrid silica nanoparticles coated with NBDAE- andSP-functionalized PNIPAM brushes immediately upon 365 nm UV irradiation for 4 min.Reprinted with permission from ref. 68. Copyright 2009, American Chemical Society.

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the temperature increases. The average FRET efficiency increases from 36%to 62% over the 20–37 1C temperature range.

Fig. 8 illustrates this change, evident both in the fluorescence spectra andin the ratio of MC to NBDAE fluorescence intensities for the UV-irradiatednanoparticles. The nanoparticle architecture is crucial to the success of thisratiometric fluorescence temperature sensor. When FRET in polymers notattached to the nanoparticles was studied, the temperature range over whichthe fluorescence exhibited significant changes was much more narrow andless useful as a sensor.

The authors note that the PNIPAM composition can be modified byinclusion of hydrophilic or hydrophobic monomers to tune the temperaturerange that can be detected with this fluorescence sensor.

4 Conclusions

Published reports of photochromic nanoparticles have progressed fromobservations of photochromic conversions in nanoparticle systems to stu-dies that harness the photoinduced change in molecular properties forincreasingly sophisticated applications. Most organic nanoparticles thathave been non-covalently doped or covalently functionalized with photo-chromes have been designed for fluorescence modulation. State-of-the-artwork on these photoswitchable fluorescent nanoparticles involves usingthem in ultrahigh resolution imaging of biological systems.

Inorganic nanoparticles functionalized with organic photochromes havebeen used for a much wider variety of applications, including not onlyimaging using fluorescence modulation but also disappearing inks that relyon photoinduced aggregation and fluorescent thermometers based onphotoinduced changes in particle dimensions, to name just a few examples.With well-established photochromic families that are synthetically andcommercially accessible, there are few limits on the creation of new pho-tochromic nanoparticle systems. The future will likely see particles forfluorescence modulation with improved brightness and switching propertiesas well as a continued increase in the number of novel applications ofphotochromic nanoparticles.

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