Light-Responsive Block Copolymer Micelles - Macromolecules (ACS

Perspective pubs.acs.org/Macromolecules

Light-Responsive Block Copolymer Micelles Yue Zhao* Département de chimie, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1 ABSTRACT: The association state of light-responsive block copolymer (BCP) micelles in aqueous solution can be altered, often reversibly, by light. Driven by the potential application in controlled drug delivery, this type of stimuli-responsive polymer micelles has received increasing attention. This Perspective highlights the progress achieved in recent years. On the one hand, we discuss the different approaches of rational BCP design, making use of various photochromic moieties and photochemical reactions, and the underlying mechanisms leading to photoinduced disruption of BCP micelles. On the other hand, we suggest possible future directions in this area, including exploration of new mechanisms and chemistry and solutions to the excitation wavelength problem crucial for biomedical applications.

1. INTRODUCTION Since the report in 2004 on a block copolymer (BCP) micelle undergoing reversibly disassembly and assembly upon ultraviolet (360 nm) and visible light (440 nm) exposure, respectively,1 there has been growing interest in designing and studying BCP micelles whose chain association state can be controlled or changed by light. The main driving force for this development in the broad area of stimuli-responsive BCP self-assembled structures is the potential for controlled drug delivery applications. If the release of drugs loaded in BCP micelles on target sites can be triggered by absorption of photons, the possibilities of remote activation as well as light-enabled spatial and temporal control are attractive features. Moreover, rational design of lightresponsive BCP micelles by exploring the photochemistry also offers interesting challenges from an academic point of view. There are several reviews that appeared in recent years either totally or in part dedicated to this topic, in which details on the studies reported so far in the literature can be found.2−13 This Perspective is meant to discuss our current understanding of the approaches proved useful for making light-responsive BCP micelles and the underlying mechanisms and to share reflections with readers on the possible future developments. It is to be stressed that the focus of this paper is set only on micellar aggregates of amphiphilic BCPs, while other types of photoresponsive polymers and related materials are not covered.

Figure 1. Schematic illustration of various types of light-responsive block copolymer micelles (a−d).

based on optically shifting the hydrophilic−hydrophobic balance of BCPs is the most studied one (Figure 1a). Generally, a photochemical reaction is made to occur inside the micelle, resulting in an increase in the polarity (or water solubility) of the hydrophobic block. This change may shift the hydrophilic− hydrophobic balance toward the destabilization of the micelle

2. TYPES OF LIGHT-RESPONSIVE BCP MICELLES Before discussing the chemical approaches employed for making BCP micelles responsive to light, a classification may be helpful. As schematically illustrated in Figure 1, light-responsive BCP micelles can be regrouped into four types based on their photoinduced structural changes. For lightening the discussion, only core−shell micelle with a hydrophobic core is shown and will generally be used in the discussion, while knowing that all the chemical approaches can, in principle, be applied to other types of BCP self-assembled aggregates such as vesicles. The first approach © 2012 American Chemical Society

Received: January 13, 2012 Revised: February 16, 2012 Published: February 28, 2012 3647

dx.doi.org/10.1021/ma300094t | Macromolecules 2012, 45, 3647−3657

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Figure 2. (a) Schematic illustration of photoinduced shifting of the hydrophilic−hydrophobic balance without removal of photochromic moieties. (b−e) Examples of block copolymer structures containing respectively azobenzene (ref 1), spiropyran (ref 33), dithienylethene (ref 38), and diazonaphthoquinone (ref 43).

photoreaction results in no removal of photochromic side groups, along with examples of reported BCP structures. Again, for the sake of clarity, no terminal and block junction groups are shown in the BCP structures unless otherwise stated. Several photochromic molecules have been used in this design, including azobenzene,14−32 spiropyran,33−37 dithienylethene,38,39 diazonaphthoquinone (DNQ),40−43 and stilbene.44 Most of them display a reversible photoisomerization reaction upon ultraviolet (UV) and visible light absorption, including the trans−cis isomerization with azobenzene and stilbene, the isomerization of spiropyran to merocyanine, and the conversion between ringopen and ring-closed forms of dithienylethene. In such cases, the UV-induced dissociation effect on BCP micelles could be reversed by visible light under appropriate conditions. By contrast, DNQ displays a photoinduced Wolf-rearrangement reaction that is irreversible. The key to photoinduced dissociation of BCP micelles is that the isomer formed upon UV irradiation should have a significantly higher polarity than the more stable isomer form. This polarity switching is most evident between neutral spiropyran and charged merocyanine. However, as a photoswitch, spiropyran is much less stable than azobenzene, for which the cis isomer may have a much higher dipole moment than the trans form with appropriate substitution.14 Figure 3 shows the BCP design in which the photoreaction results in removal of the photochromic side groups. As can be seen from the examples of BCP structures, the explored chromophores include pyrene,45,46 o-nitrobenzyl,47−53 and coumarin.54,55 The hydrophobic blocks have the common feature of possessing an aryl methylester group linked to the chain backbone. In all cases, the photoreaction cleaves the photochromic moieties and converts the hydrophobic block into a hydrophilic one by forming carboxylic acids. This BCP structural change obviously

and thus lead to its disassembly in aqueous solution. If the photoreaction is reversible, the initial balance can be restored upon exposure to light at a different wavelength, and the micelle can be reassembled in solution. In the second approach (Figure 1b), a photoreaction simply cleaves the junction of the hydrophilic and hydrophobic blocks, giving rise to the micellar disruption. The third approach (Figure 1c) consists in placing photocleavable units repeatedly along the main chain of the hydrophobic block, which ensures fast photoinduced degradation of the BCP micelle. The last approach (Figure 1d) explores a reversible photo-cross-linking reaction for all-optical stabilization and destabilization of BCP micelles. It is particularly useful in cases where the BCP micellar stability is required. Photoinduced crosslinking of associated polymer chains can then provide the structural integrity for the micelle, while a subsequent photoinduced destabilization is made possible upon polymer chain decrosslinking under a different wavelength. What follows is a discussion on each type of light-responsive BCP micelles. 2.1. Shifting Hydrophilic−Hydrophobic Balance. The majority of studies on light-responsive BCP micelles fall into this category. Basically, photochromic moieties are incorporated into the BCP structure as side groups of the hydrophobic block. Upon exposure of the micellar solution to light, the photoreaction, by design, should increase significantly the polarity of the hydrophobic block or simply convert it into a hydrophilic one. If this happens, the hydrophobic block may no longer be hydrophobic enough to retain the micellar association; consequently, the micellar aggregation may fall apart with BCP chains dissolved in aqueous solution. This design strategy can be further divided into two types of photoreactions: either without or with removal of photochromic moieties from the BCP structure upon the photoreaction. Figure 2 shows a schematic illustration of the BCP design where the 3648


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Figure 3. (a) Schematic illustration of photoinduced shifting of the hydrophilic−hydrophobic balance with removal of photochromic moieties. (b−d) Examples of block copolymer structures containing respectively pyrene (ref 45), o-nitrobenzyl (ref 47), and coumarin (ref 54).

2.2. Breaking Block Junction. Using light to break the junction unit linking the hydrophilic and hydrophobic blocks is another means to disrupt BCP micelles, being demonstrated by several groups. Figure 4 shows a schematic illustration of this approach as well as examples of reported BCP structures (in this case, the photosensitive junction unit is shown). The most popular photocleavable junction is o-nitrobenzyl-based moiety.60−65 Other photochromic moieties investigated include truxillic acid derivatives66 and inclusion complex of azobenzene with cyclodextrin (CD).67 The latter is particularly noticeable due to its reversibility. The junction between the two blocks is formed when the azobenzene group at the end of one block is in the elongated trans form that complexes with the terminal CD functionality of the other block. Upon UV light irradiation inducing the trans−cis isomerization, the junction point is broken because the bent cis isomer of azobenzene is released from the cavity of CD, while the junction can be formed again upon visible light irradiation that induces the reverse cis−trans isomerization. Despite the obvious photoinduced disruption of BCP micelles, this approach may not be the most suitable for photocontrolled release of hydrophobic species from the micelle core. As pictured in Figure 1, breaking the block junction basically removes the hydrophilic corona from the hydrophobic core. While hydrophilic chains become dissolved in aqueous solution, micelle cores should remain mostly intact in the form of hydrophobic nanoparticles. By contrast, if the BCP self-assembles into vesicles with hydrophobic chains forming the membrane, the disruption effect arising from the removal of hydrophilic blocks may be more prominent. Without the support of hydrophilic corona, the curved vesicle wall may be unstable and fragmented into small pieces of hydrophobic nanoparticles. Evidence of photoinduced release from BCP vesicles was shown in a number of recent reports.62,64 2.3. Main Chain Degradation. Instead of having one single photobreakable junction between the hydrophilic and hydrophobic blocks, photocleavable moieties can be inserted repeatedly into the main chain of the hydrophobic, micelle core-forming block. With this BCP design, fast photoinduced degradation of BCP micelles can be achieved. This approach has been explored recently with a couple of ABA triblock copolymers whose chemical structures are shown in Figure 5.68,69 In both cases, the end block A is the hydrophilic PEO and the middle block B is hydrophobic and photodegradable. In the

shifts the hydrophilic−hydrophobic balance toward the destabilization of BCP micelles. Of the photochromic moieties explored, while pyrene undergoes a photosolvolysis reaction requiring the presence of water or a protonic solvent,56 the photolysis of o-nitrobenzyl is an intramolecular rearrangement reaction that needs no water57 and can be activated not only by UV light but also by near-infrared (NIR) light through two-photon absorption.58 The particular merit of coumarin resides in the fact that it is has a more efficient two-photon absorption of NIR light than o-nitrobenzyl derivatives.59 This wavelength issue is important to address for biomedical applications as will be discussed later in the paper. It is important to point out that a photoinduced shift of the hydrophilic−hydrophobic balance of BCPs cannot always result in straightforward dissociation or disassembly of micelles as sketched in Figure 1. A more appropriate term describing the possible photoinduced effects on BCP micelles reported in the literature would be photoinduced disruption that includes dissociation, swelling, and morphological transitions. This is understandable. In principle, to make the BCP micelle dissociate in water, the photoinduced shifting of the hydrophilic− hydrophobic balance should bring the amphiphilic BCP to below its critical micelle concentration (CMC), which generally is very small for polymers. The magnitude of the photoinduced change in the CMC of a given BCP is not only determined by the photoinduced polarity change but also coupled to other structural parameters such as the chain lengths (molecular weights) and the BCP composition (the relative contents of the blocks). The kinetics of polymer chain rearrangement in solution could also play a role in the observed photoinduced effect. For instance, if the hydrophobic polymer has a high glass transition temperature, Tg, or is crystallized, the dissociation of the glassy or crystalline micelle core may be slow to proceed despite a photoinduced increase in polarity. Moreover, some seemingly trivial and often unreported details on the experimental conditions, such as BCP concentration, micellar solution volume, stirring rate, and excitation light intensity, are actually important for determining the observable photoinduced effect on BCP micelles. In the context of controlled drug delivery applications, regardless of the photoinduced disruption process, what may be important is that the disruption of micelles can bring the payloads or expose them to the aqueous medium where they could encounter or interact with the target or relevant biomolecules. 3649


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Figure 4. (a) Schematic illustration of photoinduced breaking of the junction between the hydrophilic and hydrophobic blocks. (b−d) Examples of block copolymer structures whose photolabile junction is based on respectively o-nitrobenzyl (ref 63), truxillic acid derivative (ref 65), and inclusion complex of azobenzene and cyclodextrin (ref 67).

Figure 5. (a) Schematic illustration of photoinduced main chain degradation. (b, c) Examples of ABA-type triblock copolymer block structures whose hydrophobic central block contains a photocleavable o-nitrobenzyl moiety in each repeating unit (refs 68 and 69, respectively).

a photocleavable o-nitrobenzyl group but also a redox-cleavable disulfide functionality in each repeating unit.69 With this design, BCP micelles can undertake either fast photodegradation or slow degradation induced by a reducing agent. Much remains to be explored by using this approach. For instance, BCP vesicles undergoing fast photoinduced disintegration can be expected. In principle, other photolysis reactions leading to photodegradation of polymer main chains can also be employed in the BCP design. For example, polymers comprising ketal or acetal repeat units

first case, the middle block, prepared by polycondensation, possesses a photocleavable o-nitrobenzyl group in each repeating unit so that it can be cut into small segments under UV light irradiation. With BCP micelles in aqueous solution, this feature leads to fast photodegradation of the hydrophobic micelle core and thus allows for burst release of loaded species,68 which is of interest for photocontrolled drug delivery. In the second case of the ABA triblock copolymer, the middle block, prepared by click condensation, was designed to have not only 3650


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Figure 6. (a) Schematic illustration of reversible photoinduced cross-linking and de-cross-linking of block copolymers using a reversible photoreaction at two different wavelengths. (b) Chemical structure of a block copolymer containing a number of coumarin side groups that can undergo reversible photodimerization (for cross-linking) and photocleavage (for de-cross-linking) (ref 78). (c, d) Examples of coumarin-containing block copolymer structures of which one or two blocks have a lower critical solution temperature (LCST) (refs 83 and 84, respectively).

photoresponsiveness and thermal switching capability.83−87 As seen from the examples of BCPs in Figure 6, the basic design is to have one or two blocks that display a temperature-dependent water solubility characterized by a lower critical solution temperature (LCST) (soluble at T < LCST and insoluble at T > LCST) and that bear a small amount of coumarin side groups for photoinduced chain cross-linking. In a study using the diblock copolymer composed of PEO and coumarincontaining poly[2-(2-methoxyethoxy)ethyl methacrylate] (PMEO2MA) (Figure 6c),83 after dissolving the BCP in cold water, core-cross-linked micelles are easily obtained by heating the solution to T > LCST of PMEO2MA and exposing the solution to UV light at λ > 310 nm. Upon subsequent cooling to T < LCST, the cross-linking prevents the micelle from dissolution, which gives rise to nanogel particles with both watersoluble core and shell as well as phototunable size by adjusting the cross-linking density through the decross-linking using UV light at λ < 260 nm. In another study using a BCP composed of coumarin-containing poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) and poly(N-isopropylacrylamide) (PNIPAM) (Figure 6d),84 large vesicles with dehydrated PNIPAM membrane are formed at T > LCST of PNIPAM and subjected to corona cross-linking through dimerization of coumarin. Upon cooling to T < LCST, vesicles undergo expansion with up to 700% increase in the hydrodynamic volume due to the hydration of PNIPAM membrane in cross-linked vesicles retaining their structural integrity, while contraction occurs upon subsequent heating to T > LCST. After photoinduced de-cross-linking, vesicles are dissolved at T < LCST of PNIPAM. Other photochromic molecules exhibiting a reversible photodimerization reaction, such as cinnamic acid,88−91 can also be utilized for reversible cross-linking of BCP micelles.

in their main chains were shown to undergo UV light-induced degradation at low energies.70 2.4. Reversible Cross-Linking. Some BCP micelles may be instable in the body and fall apart due to dilution of the BCP to below its CMC or interactions with biomolecules (e.g., enzymatic degradation).71 This micellar instability can result in undesired premature release of drugs. To combat this problem, a long-known strategy consists in chemically cross-linking polymer chains constituting the micelle so that they cannot be separated from each other, which provides the structural integrity of the micellar association, could reduce the diffusion of payloads into the solution and also prolong the circulation time in the blood.72−77 However, such enhanced micellar stability has a conflicting effect because when it is time to release the payloads, the chain cross-linking can also make their release more difficult.77 In such cases, the approach based on using a reversible photo-cross-linking reaction is particularly useful. The idea is to first stabilize BCP micelles by cross-linking using light at one wavelength and then to destabilize them by breaking the crosslinking using light at a different wavelength.78 The most used reversible photo-cross-linking reaction is the photodimerization through cycloaddition of coumarin groups under UV light at >310 nm and the subsequent cleavage of cyclobutane bridges under UV light at LCST-1 leading to formation of BCP micelles. Upon exposure to light, if the photoreaction makes the chromophore-containing units more polar, the LCST of the block may shift to a higher temperature LCST-2 so that T < LCST-2 (solution temperature below the new hydration−dehydration transition temperature); consequently, the block turns to be soluble in water and the micelle is dissolved. Some studies have already confirmed this effect with either irreversible photocleavage of o-nitrobenzyl methylester groups48 or the reversible trans−cis photoisomerization of azobenzene.31 More research efforts should be made to explore this mechanism because potentially it can provide a means for amplifying the effect of a photoreaction. Indeed, if the photoreaction of a very small amount of photochromic groups can lead to a significant increase in the LCST of the micelle coreforming block, the low content of photochromic triggers may be particularly appealing for possible biomedical applications. In other mechanisms allowing for photoinduced disassembly of BCP micelles, generally, each monomeric unit of the photosensitive block brings a photochromic moiety, while a high content of photochromic molecules is a concern for cyctotoxicity. However, achieving a large LCST increase with a very small amount of photochromic groups is challenging, particularly for reversible LCST switching.100 For this reason, continuing research efforts aimed at understanding how the result of a photoreaction can impact the LCST of a polymer 3652


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and at discovering new mechanisms of phototunable LCST101 is necessary. The fast progress on light-responsive BCP micelles in recent years has been possible thanks to the advances in the versatile and robust controlled radical polymerization techniques, such as atom transfer radical polymerization (ATRP) and reversible addition−fragmentation chain transfer polymerization (RAFT), which made the syntheses of most designed photosensitive BCPs easily accessible. Future development in this area will still benefit from new polymer synthetic methods. Most notable examples include the use of click chemistry102 and controlled ring-opening methathesis polymerization (ROMP)103,104 to prepare polymers with photochromic moieties incorporated in the structures. Polymerization of chromophore-containing monomers via ATRP or RAFT generally yields polymers with limited molecular weights. Quantitatively linking photochromic molecules to a polymer of high molecular weight using the click coupling reaction between azide and alkyne groups provides a way of producing high molecular weight photosensitive polymers.102 In addition to high molecular weights, ROMP could offer a better control of the polydispersity.103,104 Photosensitive BCPs of well-defined architectures or high molecular weights are interesting to investigate because these are parameters that affect the BCP self-assembly. 3.2. Extension to Biocompatible and Biodegradable Micelles. The various BCP design strategies for lightresponsive micelles are now well established; it is time to extend their use to biocompatible and biodegradable polymer micelles since, as mentioned above, the main interest of this type of materials is to develop an optical modality for controlled drug delivery applications. Most studies reported so far have been focused on proving the concept or principle while taking little care of the toxicity of the polymers. Examining the structures of BCPs in the literature, it is easy to notice that PEO or poly(ethylene glycol) (PEG) is generally utilized as the hydrophilic block. This is a good choice because a corona of PEO surrounding the micelle core ensures the solubility or dispersion of micelles in aqueous solution, enables steric stabilization, and can inhibit the adsorption of biological substances on the micelle surface.71 The photochromic moietycontaining hydrophobic blocks are often polymethacrylates or polyacrylamides, which are among the polymers much studied as biomaterials. Nevertheless, future studies of photosensitive BCP micelles using these polymers should pay more attention to the toxicity issue. A more straightforward approach to preparing biocompatible light-responsive BCP micelles is probably to combine the hydrophilic PEO with hydrophobic poly(amino acids) such as poly(aspartic acid) (PAsp) and poly(glutamic acid) (PGlu) while applying the design principles discussed above. Some recent works have already taken action in this direction. In one case, a diblock copolymer composed of PEO and PGlu containing a number of spiropyran side groups was synthesized, and its micelle could be reversibly disassembled and reassembled upon UV and visible light irradiation, respectively, due to the spiropyran−merocyanine photoisomerization switching the polarity of the hydrophobic block.37 In another case, irreversible photoinduced disassembly of micelles of a BCP comprising PEO and PGlu bearing coumarin pendant groups was achieved using NIR two-photon absorption induced cleavage of the photochromic groups, resulting in light-triggered release of antibacterial or anticancer drugs.55 In addition to the choices of hydrophilic and hydrophobic polymers for the constituting blocks, the choice

of the photochromic moieties whose photoreactions trigger the micellar disruption may also be a factor to minimize the toxicity. However, it can be noticed that most of the photochromic molecules and their derivatives found in the reported light-responsive BCP micelles have been often used in biomolecules or biomaterials.105−107 Of course, any research efforts on evaluating light-responsive BCP micelles in designed in vitro or in vivo studies are timely. Light-responsive BCP micelle−drug conjugates are largely unexplored and may represent an interesting future direction. In a recent report,108 an anticancer drug, 5-fluorouracil, is linked covalently to coumarin side groups on the hydrophobic block of a diblock copolymer through UV-induced cycloaddition (>310 nm) and subsequently released from micelles formed by the drug-functionalized BCP under shorter-wavelength UV light (254 nm). The disassembly of BCP micelles by converting the hydrophobic block into a hydrophilic one through photocleavage of photochromic moieties (Figure 3) basically requires an aryl methylester group.56 It is conceivable that some hydrophobic drugs have aromatic rings and can be attached to the hydrophobic block through a methylester unit. In this way, the drug-functionalized block should form the core of micelle, allowing for the encapsulation of a large amount of drug; when exposed to light, the release of drug molecules could occur simultaneously with the disassembly of BCP micelles. 3.3. The Wavelength Issue: Near-Infrared Light Is Better. For most biological or biomedical applications of photosensitive materials (including controlled drug delivery), the excitation or photoreaction of photochromic moieties by absorption of long-wavelength NIR light (roughly between 800 and 1000 nm) is preferred to UV or visible light. As compared to UV or visible light, NIR light is less detrimental to healthy cells and can have a deeper penetration through tissue (in the order of magnitude of centimeters) due to its reduced absorption and scattering by water and biological substances.59,109,110 Since most light-responsive BCP micelles require UV or visible light for the photoreactions responsible for the desired control or disruption of micelles, the wavelength has always been an issue that hampers their possible applications. Knowing that a solution to this wavelength problem is to use two-photon absorption of NIR light to activate the photoreactions, providing the same light energy as one-photon absorption in the UV or visible spectral region, a significant amount of research efforts have been dedicated to developing NIR light-sensitive BCP micelles40,47,54,55 and other polymeric materials.98,99 Among the chromophores exhibiting two-photon absorption of NIR light, including o-nitrobenzyl esters and coumarin derivatives, coumarin-based moieties are more efficient for NIR light-induced micellar disruption due to a greater two-photon absorbing cross section.59 Future researches on NIR light-responsive BCP micelles via two-photon absorption should find a mechanism allowing the effect due to NIR light-induced removal of a few photochromic moieties to be amplified leading to micellar disassembly. As discussed above, a possible approach is to develop polymer structures of which the LCST can be increased greatly by the photoreaction of a few photochromic groups (Figure 7b). Although using two-photon absorption of NIR for lightresponsive BCP micelles is interesting and worth continuing effort, there are a number of constraints related to it. Not all photochromic molecules have usable two-photon absorption efficiencies. Even for those having been used to this end, the 3653


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Figure 8. Schematic illustration of encapsulating upconversion nanoparticles (UCNPs) in photosensitive block copolymer micelles and exciting them with continuous-wave near-infrared (NIR) light for emission of UV or visible light that, in turn, activates the photoreaction leading to the disassembly of micelles and release of payloads (adapted with permission from ref 111).

using this strategy to develop NIR light-responsive BCP micelles can be expected.

photoreactions activated by two-photon absorption of NIR light are generally slow and inefficient due to the typically low two-photon-absorbing cross sections of the chromophores. For some photoreactions discussed above, for example, the reversible dimerization of coumarin and cleavage of cyclobutane (typically under ∼310 and 260 nm UV light irradiation, respectively), even doubled via two-photon absorption, the wavelengths still fall into the visible region. Technically, since the simultaneous absorption of two photons necessitates high laser power density, the use of a femtosecond pulse laser is required. In order to make NIR light-triggered drug release a more viable modality, more general, robust, and accessible strategies of using NIR light need to be developed, not only for complementing the two-photon absorption approach but also for overcoming its drawbacks. An example of such new strategies has been demonstrated recently which enables the use of continuous-wave NIR light with a diode laser.111 As schematized in Figure 8, the strategy makes use of lanthanidedoped upconverting nanoparticles (UCNPs) that can absorb NIR light and convert it to higher-energy photons in the UV and visible regions.112−115 Since the excitation of UCNPs by NIR light occurs via sequential, multiple absorptions with real energy levels, it requires much lower power density (by several orders of magnitude) than two-photon absorption and a continuous-wave diode NIR laser can thus be sufficient as the excitation source. By encapsulating NaYF4:TmYb UCNPs inside photosensitive micelles of a diblock copolymer composed of PEO and a polymethacrylate bearing o-nitrobenzyl side groups, and exposing the micellar solution to 980 nm NIR light, photons in the UV region (∼350 nm) emitted by UCNPs inside the micelles are absorbed by the photochromic moieties on the micelle core-forming block, which activates the photocleavage reaction (Figure 3c) and leads to the dissociation of BCP micelles and release of payloads.111 In this strategy, continuous-wave NIR is the excitation light while UCNPs loaded in micelles act as an internal UV or visible light source that executes the target photoreaction. It is general because all the photoreactions requiring the use of UV or visible light and explored for all types of light-responsive BCP micelles can be activated by NIR light via excitation and emission of UNCPs. In view of the fast development of UCNPs with tunable particle sizes and emission wavelengths,116,117 exciting future studies

4. FINAL REMARKS Significant progress in the design and preparation of lightresponsive BCP micelles has been made in recent years. However, there are still many challenges from both academic and application points of view. Ultimately, for light-triggered delivery of drugs or other bioagents, one needs to develop biocompatible BCP micelles that have low cytotoxicity, can contain a large amount of payloads, are stable in the blood for a long circulation time without premature leakage, can selectively accumulate in diseased tissues or cells, and, then, can be disrupted by NIR light in a controlled or even quantitative manner (for example, fast disassembly leading to burst release in cancer cells). To achieve this level of functions and control, obviously much research remains to be done in the future. For the many UV- or visible light-sensitive BCP micelles, it may be interesting to start exploring their possible applications in other areas such as cosmetic and agricultural industries where the necessity of using NIR light is not a concern.118 Finally, as mentioned at the beginning of this article, only light-responsive amphiphilic BCP micelles are discussed and highlighted. Similar approaches and strategies can be found in other photoresponsive materials and macromolecular architectures including dendritic polymers,119−123 hyperbranched polymers,124−126 hydrophobically modified polymers,127−129 hydrogels,130−132 polymer capsules,118,133 supramolecular assemblies,134−136 amphiphilic random copolymers,137,138 and small-molecule surfactants.139−146 Some discussed new developments on BCP micelles will also have repercussion on those systems. For instance, the use of continuous-wave NIR light excitation of UCNPs to activate the photoreactions requiring UV or visible light should be an effective way to resolve the wavelength issue for many other photosensitive materials developed for biological and biomedical applications.

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*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 3654


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Yue Zhao received his B.S. degree in 1982 from Chengdu University of Science and Technology (now Sichuan University) in China. In 1983, he went to France and studied at École Supérieure de Physique et de Chimie Industrielles de Paris with Prof. Lucien Monnerie. After obtaining his PhD in 1987, he did postdoctoral studies with Prof. Robert Prud’homme at Laval University, Quebec, Canada. In 1991, he joined the Chemistry Department of University of Sherbrooke and has been a full professor since 2000. His current research focuses on the design, synthesis, and study of stimuli-responsive, self-assembled, and nanostructured polymer and liquid crystal materials.

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ACKNOWLEDGMENTS I am grateful to all my students and collaborators who have made contributions to the research works conducted by my group on light-responsive block copolymer micelles, particularly, Drs. Jinqiang Jiang, Guang Wang, Jérôme Babin, Mrs. Xia Tong, Drs. Dehui Han, Jie He, Yi Zhao, Surjith Kumar, and Mr. Bin Yan. I also want to express my gratitude for the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), le Fonds québécois de la recherche sur la nature et les technologies of Québec (FQRNT), Université de Sherbrooke, St-Jean Photochemicals Inc. (St-Jean-sur-Richelieu, Québec, Canada), The Cancer Research Society Inc. (Canada), and the FQRNT-funded Center for Self-Assembled Chemical Structures (CSACS). I thank Drs. Surjith Kumar, Dehui Han, and Mr. Bin Yan for their assistance in preparing the illustrations, figures, and references used or cited in this article.

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Light-Responsive Block Copolymer Micelles - Macromolecules (ACS

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