Hybrid photosensitizer based on amphiphilic block copolymer stabilized silver nanoparticles for highly efficient photodynamic inactivation of bacteria

Abstract

We report the development of a type of novel hybrid photosensitizers for photodynamic inactivation of broad-spectrum bacteria. A thiol-modified amphiphilic block copolymer, poly(N-isopropylacrylamide-block-styrene), was synthesized and characterized. Subsequently, silver nanoparticles stabilized by poly(N-isopropylacrylamide-block-styrene) were synthesized and used to entrap hydrophobic photosensitizing molecules (such as hematoporphyrin). The resulting water-dispersible hybrid photosensitizers demonstrated enhanced singlet oxygen generation with a broadened excitation profile, as compared to the pristine hematoporphyrin molecules. Photodynamic inactivation of Staphylococcus epidermidis and Escherichia coli by the hybrid photosensitizer showed significantly enhanced killing efficacy, up to ∼5 orders of magnitude, under both white light and red/near-infrared light illuminations. The hybrid photosensitizers at the concentration used in the photodynamic inactivation assays displayed low cytotoxicity to Hela cells under ambient light conditions. These results demonstrate the great potential of such hybrid photosensitizers for photodynamic inactivation and photodynamic therapy applications.

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1. Introduction

Molecular oxygen at the ground state is a unique triplet spin electronic state (³Σ_g⁻). The two low-lying singlet excited states, ¹Δ_g and ¹Σ_g⁺, are 95 and 158 kJ mol⁻¹ above this triplet state, respectively. The lower-energy singlet excited state of molecular oxygen, ¹Δ_g, is commonly referred to as singlet oxygen (¹O₂), and has a relatively longer lifetime compared to the ¹Σ_g⁺ state. Singlet oxygen is a highly reactive species playing a crucial role in a variety of chemical and biological processes. First observed in 1924, singlet oxygen has drawn intensive attention in recent decades.^1–3 The ¹Δ_g to ³Σ_g⁻ transition at ∼1280 nm can be spectrally observed and used as a direct proof of the existence of singlet oxygen.⁴

A convenient and controllable way to generate singlet oxygen is photosensitization. The most important application for photosensitized singlet oxygen is photodynamic therapy (PDT), which is increasingly used in cancer treatments.^5–7 The three components of photodynamic therapy are oxygen, light with appropriate wavelengths, and photosensitizers. When combined, these components are shown to generate singlet oxygen, causing damage to tumor cells locally, which tends to incur fewer side effects as compared to chemotherapy or radiation therapy.⁸ Photodynamic therapy has also been identified as a viable approach for photodynamic inactivation of bacteria (PDI) in recent years.^9–11 The advantage of using PDI to combat drug-resistant bacteria lies in the fact that reactive oxygen species (ROS), such as singlet oxygen, is highly reactive toward antioxidant enzymes and difficult for bacteria to defend against, regardless of their drug-resistance status.^9,12 However, PDT and PDI suffer from some disadvantages that limit their use. For instance, most photosensitizing molecules have moderate efficiency of singlet oxygen generation and many are hardly soluble in aqueous media; they are usually excited by blue or green light, of which the tissue penetration depth is limited. Innovative approaches are needed to improve photosensitizers for PDT and PDI applications.

In this regard, a lot of efforts have been made to increase singlet oxygen generation efficiency. Synthesis of organic photosensitizing molecules with absorption band at red or near-infrared (NIR) region is one of the solutions^13,14 despite some drawbacks, such as, time-consuming, complicated processes, and moderate singlet oxygen generation efficiency.² Meanwhile, recent studies have shown that singlet oxygen generation can be enhanced when the photosensitizing molecules are placed in proximity to metal nanoparticle surface.^15–18 In particular, resonance coupling between photosensitizing molecules and metal nanoparticles, associated with the spectral overlap between the absorption band of the photosensitizing molecules and the plasmonic band of the metal nanoparticles, appears to greatly enhance the singlet oxygen generation, leading to hybrid photosensitizers with high PDT and PDI efficacy.^19,20 To the other front, core–shell nanoparticle-copolymer structures have been used as a platform to encapsulate hydrophobic molecules, such as many drug molecules, in the hydrophobic segment of copolymers for potential drug delivery.^21,22

Here we reported a novel platform to synthesize hybrid photosensitizers based on amphiphilic block copolymer-stabilized silver nanoparticles (AgNPs) and hydrophobic photosensitizing molecules. The resulting hybrid photosensitizers display highly enhanced singlet oxygen generation, as evidenced by spectroscopic measurements, and PDI efficacy, as demonstrated by in vitro antibacterial tests. The excitation profile of the hybrid photosensitizers is broadened by the presence of AgNPs, allowing the PDI tests to be carried out under red/NIR illumination. The platform also provides a means to use many photosensitizing molecules, which are insoluble in water and thus difficult to be used in applications involving aqueous media.

2. Experimental

2.1 Chemicals and reagents

Silver nitrate (99%), hematoporphyrin (HP, 45%), N-isopropylacrylamide (NIPAAm, 99%) and 2,2′-azobis(2-methylpropionitrile) (AIBN, 99%), Minimum Essential Media (MEM), and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide were purchased from Sigma Aldrich. NIPAAm was recrystallized in hexane. AIBN was recrystallized in methanol. 2-(Dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) was synthesized following procedures in ref. 23. Sodium borohydride was purchased from Fluka. Hematoporphyrin (HP) was from Frontier Scientific. Phosphate buffered saline (PBS, 10× solution), granulated agar and penicillin–streptomycin were purchased from Fisher Scientific. Fetal calf serum (FBS) was purchased from Hyclone. Mueller Hinton II broth, trypticase soy broth and nutrient broth were purchased from Becton Dickinson. Staphylococcus epidermidis (ATCC 35984), Escherichia coli (ATCC 35218) and Human Negroid cervix epithelioid carcinoma cell line (Hela) were purchased from American Type Culture Collection (ATCC, USA).

2.2 Synthesis of poly(N-isopropylacrylamide-block-styrene) (PNIPAAm-b-styrene)

2.2.1 Synthesis of PNIPAAm. NIPAAm (12.00 g, 0.106 mol), DDMAT (0.193 g, 0.53 mmol) and AIBN (4.34 mg, 26 μmol) were added to 48 mL 1,4-dioxane in a round-bottom flask equipped with a stir bar. The reaction flask was sealed with a rubber septum and purged with N₂ for 30 min in an ice bath. The reaction was allowed to proceed at 60 °C for 3 h and stopped by exposing the reaction to air. The residue was then precipitated into cold ether three times to yield 6.00 g of yellow solids. ¹H and ¹³C NMR spectroscopy were performed in CDCl₃, with Si(CH₃)₄ standard, using a 400 MHz Bruker Ultrashield (100 MHz for ¹³C). ¹H NMR and ¹³C NMR spectra were analyzed with MestReNova software. Molecular weights of polymers were determined using an Agilent 1100 Series HPLC equipped with a DMF mobile phase and Optilab T-rEX differential refractometer (light source at 658 nm) (Wyatt Technology Corporation) detector. Polymer was characterized using gel permeation chromatography: number-average molecular weight (M_n) = 15.9 kDa, dispersity (Đ) = 1.25. The ¹H NMR spectrum of PNIPAAm is shown in Fig. S1.†

2.2.2 Synthesis of PNIPAAm-block-styrene. 1.00 g macro-CTA, styrene (2.95 mL, 25.7 mmol) and 0.78 mg AIBN (4.67 μmol) were added to 15 mL 1,4-dioxane in a round-bottom flask equipped with a stir bar. The reaction flask was sealed with a rubber septum and purged with nitrogen for 30 min. The reaction was allowed to proceed at 80 °C for 3 h and stopped by exposing the reaction to air. The residue was then precipitated in cold ether three times to yield yellow solids (0.97 g). Polymer was characterized using gel permeation chromatography: M_n = 20.5 kDa, Đ = 1.65. Sodium borohydride (NaBH₄) was used to reduce trithiocarbonylthio groups to thiols in the polymers. NaBH₄ (0.8 g) and 0.8 g polymer were dissolved in 30 mL of THF and 10 mL of water was added. The reaction was allowed to proceed at 25 °C overnight. The residue was then precipitated in cold ether three times to yield the final polymer as white power (0.50 g). The amphiphilic block copolymer (BCP) was characterized using GPC: M_n = 20.1 kDa, Đ = 1.98. Proton NMR spectrum of PNIPAAm-b-styrene is shown in Fig. S2.†

2.3 Synthesis of Ag@PNIPAAm-styrene nanoparticle (AgNP)

In a typical synthesis of the PNIPAAm-b-styrene stabilized AgNPs, 1.7 mg silver nitrate was dissolved in a mixture of 1.00 mL deionized water and 7.00 mL ethanol. Then, a 1.00 mL aqueous solution containing 10.2 mg PNIPAAm-b-styrene was added with vigorous stirring. The solution was kept stirring for 30 min to allow the formation of the silver-thiolate precursor. Afterwards, a 1.00 mL aqueous solution containing 7.6 mg sodium borohydride was added quickly. The mixture was stirred for 1 h. The product was then centrifuged at 14 000 rpm for 20 min and washed by water/ethanol (80/20).

2.4 Synthesis of AgNP@BCP@HP hybrids

The freshly prepared Ag@PNIPAAm-styrene NPs were dispersed in 8.00 mL deionized water. Then, a 2.00 mL ethanol solution containing 12.1 mg HP was added and stirred at room temperature for 48 hours. The product was centrifuged at 14 000 rpm for 20 min and washed by water/ethanol (80/20) three times to remove excess photosensitizers. The as-synthesized hybrids were dispersed into 10 mL deionized water under sonication, and stored for later use.

2.5 TEM measurement

TEM observations were performed using a Biotwin 12 transmission electron microscope. A drop of the sample aqueous solution was placed and dried onto a carbon-coated copper grid (300 mesh, EMS).

2.6 UV-vis spectral measurement and determination of HP concentration in the hybrid

A UV-vis spectrometer (USB4000-ISS, Ocean Optics) was used to measure the UV-vis absorption spectra. The concentrations of HP in all samples were determined by measuring their UV-vis absorption spectra. A series of HP water/ethanol (50/50) solutions ranged from 1 to 10 μM were prepared with the mixture of deionized water and ethanol (50/50) as solvent. UV-vis spectra of the solutions were measured to create a calibration curve as shown in Fig. S3.† To determine the loading efficiency of HP in the hybrid, the hybrid solution was mixed with an excess amount of sodium cyanide solution (100 μM) to remove the Ag core and finally diluted by ethanol to prepare a mixed water/ethanol (50/50) solution. By measuring the UV-vis spectrum of the NaCN-treated hybrid solution and comparing to the calibration curve, we determined the loading efficiency of HP in the hybrids to be 34.7 μg mg⁻¹ (HP/hybrid). This value is used to determine the concentrations of HP used in the photodynamic inactivation assays.

The possible HP leakage of Ag@PNIPAAm-styrene@HP hybrids were investigated in DI water and PBS, respectively. In the case of water, hybrids were dispersed into 5 mL of DI water under sonication and kept in dark at room temperature. After 24 h, 1 mL of the solution was centrifuged at 14 000 rpm for 20 min. The supernatant was discarded, and the pellet of hybrids was resuspended in 0.5 mL ethanol. Then, 0.5 mL aqueous solution containing neutralized sodium cyanide in excess was added to dissolve the silver core. The UV-vis absorption spectrum was collected for the resulting solution to determine the HP concentration. This procedure was repeated four more times at intervals of 24 h. Same procedures were followed to determine the HP leakage from hybrids in PBS buffer. The results are shown in Fig. S3c.†

2.7 Fluorescence and phosphorescence measurements

A QM-40 spectrofluorometer (PTI Inc.) equipped with a high performance InGaAs photodiode and a lock-in amplifier (Model 410 singlet phase, Scitec Instruments Ltd.) was used to measure the fluorescence and phosphorescence spectra. Detection of the singlet oxygen production was carried out by monitoring its phosphorescence emission at ∼1280 nm. The light source was a xenon arc lamp, of which output passed through an optical chopper operating at a fixed frequency. Samples were loaded into a quartz cuvette and placed in a light-tight chamber, with the emission signal collected orthogonal to the excitation beam. An additional long-pass filter (850 nm cut-off) was used to remove any possible higher-order artifact signals. Samples were dispersed in water/ethanol (50/50) for testing. All fluorescence and phosphorescence measurements of AgNP@PNIPAAm-styrene@HP hybrids were carried out on the same instrument. The excitation wavelength used to collect fluorescence emission spectra was 395 nm. Phosphorescence excitation spectra were taken with 1280 nm as the emission wavelength.

2.8 Photodynamic inactivation (PDI) assays

Typically, overnight cultures of S. epidermidis (ATCC 35984) and E. coli (ATCC 35218), were inoculated into PBS buffer solution (pH = 7.4) and mixed with a series of concentrations of AgNP@BCP@HP, pristine HP (dissolved in the same PBS buffer), or AgNP@BCP. All bacterial suspensions (200 μL), including non-treated controls, were then placed in the wells of 96-well plates. The final bacterial concentration of the suspensions was ∼10⁶ to 10⁷ colony-forming units per mL (CFU mL⁻¹). The wells were illuminated under different fluences. After illumination, a plate count method was used to determine the viable bacterial numbers (CFU ml⁻¹) in suspensions.²⁴ A series of AgNP@BCP solutions, which did not mix with HP, were treated through the same washing steps as the AgNP@BCP@HP hybrid solutions, to ensure they had the same concentrations of Ag as the AgNP@BCP@HP hybrid solutions. These AgNP@BCP solutions, the freshly prepared pristine HP solutions, and their respective dark controls were run in parallel. Note that AgNP@BCP@HP has the same HP concentration as the pristine HP, and the same AgNP concentration as AgNP@BCP. Three independent runs were carried out for each experiment. A non-coherent, white light source with interchangeable fiber bundle (model LC-122, LumaCare) was used in all photoinactivation experiments. A bandpass filter (Edmund Optics #84-903, absorbance spectrum shown in Fig. S6†) was used to select the red/NIR region of the light source. The light intensity of white light and red/NIR light were 408 mW cm⁻² and 231 mW cm⁻², respectively, as measured by a laser power meter (model 840011, SPER Scientific). The light exposures for S. epidermidis and E. coli under white light were 49 J cm⁻² (2 min illumination) and 294 J cm⁻² (12 min illumination), respectively. The light exposure for S. epidermidis and E. coli under red/NIR light were 139 J cm⁻² (10 min illumination) and 277 J cm⁻² (20 min illumination), respectively. Each PDI experiment was performed in triplicate. The primary data are presented as the means with standard deviations. Differences are analyzed for statistical significance by the two-sample t-test and the probability values of <5% are considered significant.

2.9 Cell viability assays

In vitro cytotoxicity of AgNP@BCP@HP was evaluated using a standard methyl thiazolyl tetrazolium (MTT) assay without light illumination. Hela cells were maintained in Minimum Essential Media (MEM) supplemented with 10% FBS, 2 mM L-glutamine, 100 units per ml penicillin and 0.1 mg ml⁻¹ streptomycin at 37 °C, in an incubator with a 5% CO₂ atmosphere. Briefly, 100 μL of Hela cells were seeded in the wells of a 96-well plate at the density of 10 000 cells per well. After overnight incubation in the incubator (37 °C, 5% CO₂), the cells were treated for 24 h with various concentrations of AgNP@BCP@HP or AgNP@BCP (both containing same amount of Ag, and hybrids containing 10 μM HP) in MEM (supplemented with 10% FBS) at 37 °C under 5% CO₂. Subsequently, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide in PBS solution (5 mg mL⁻¹) was added to the culture medium to reach a final concentration of 0.5 mg mL⁻¹. After the cells were left at 37 °C for 4 h, the supernatants were removed and the formazan dye was dissolved in 100 μL of DMSO. Absorbance was measured on a microplate reader at 490 nm with a reference wavelength at 650 nm. Each experiment was done in 6 wells in parallel.

3. Results and discussion

3.1 Synthesis of AgNP@BCP@HP

The design of the hybrid photosensitizer is illustrated in Fig. 1. The amphiphilic block copolymer used in this study is poly(N-isopropylacrylamide-b-styrene) (PNIPAAm-styrene), which contains a thiol group at the hydrophobic end of the polymer chain. PNIPAAm-b-styrene is synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization using 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid as chain transfer agents (CTA). PNIPAAm is first synthesized as a macro-CTA. Styrene is then copolymerized with this macro-CTA to afford a BCP, followed by the reduction of trithiocarbonylthio group to a thiol by sodium borohydride. PNIPAAm-b-styrene stabilized AgNPs (noted as AgNP@BCP hereafter) are synthesized directly in water/ethanol (30/70) by using sodium borohydride as the reducing agent and PNIPAAm-b-styrene as the capping agent.²⁵ Hematoporphyrin, a photosensitizing molecule with poor solubility in water, is subsequently entrapped in the hydrophobic styrene block of the polymer. The long hydrophilic PNIPAM chains not only keep HP close to the AgNP but also render the AgNP@BCP@HP hybrids dispersible in water.

Fig. 1 Illustration of synthesis of block copolymer-coated, HP-entrapped silver nanoparticles (AgNP@BCP@HP).

The BCP-stabilized AgNPs are characterized spectroscopically and by TEM. The TEM image (inset of Fig. 2a) shows that the AgNPs are rather uniform with an average size of ∼20 nm. Normalized UV-vis absorption spectra of AgNP@BCP, pristine HP and AgNP@BCP@HP are shown in Fig. 2a. AgNP@BCP has a strong absorption band at ∼400 nm, typical for AgNPs of similar sizes. Pristine HP exhibits a strong Soret band at ∼397 nm and four weak Q bands (450–650 nm). Hence, there is a significant spectral overlap between the AgNP@BCP plasmon band and the HP absorption band, suggesting the possibility of resonance coupling between HP and AgNP@BCP. The AgNP@BCP is very stable and well dispersed in aqueous solutions. The entrapped HP in AgNP@BCP@HP is also stable, as indicated in the results of 5-day leakage test (Fig. S3c in ESI†).

Fig. 2 (a) Normalized UV-vis absorption spectra of AgNP@BCP, pristine HP and AgNP@BCP@HP. Inset: TEM image of AgNP@BCP. Scale bar is 100 nm. (b) Emission spectra of AgNP@BCP@HP and pristine HP of the same concentration (0.125 μM).

3.2 Spectroscopic properties of AgNP@BCP@HP

AgNP@BCP has greatly affected the spectroscopic properties of the entrapped HP. As shown in Fig. 2b, the fluorescence intensity of HP in AgNP@BCP@HP is much lower than in pristine HP of the same concentration. Besides, while the fluorescence intensity of pristine HP solution increases proportionally with the increase of its concentration (Fig. S4b†), there is no linear relationship between the fluorescence intensity of the entrapped HP and its concentration in AgNP@BCP@HP (Fig. S4d†). This could be due to the self-quenching of the entrapped HP in AgNP@BCP@HP, especially at high concentration, as they are confined within the polymer coating on AgNP surface. There is also quenching of the entrapped HP by AgNP, as observed at the lower HP concentrations in AgNP@BCP@HP, indicating strong interaction between the entrapped HP and AgNP@BCP.

The capability of singlet oxygen generation by the entrapped HP is greatly enhanced by AgNP@BCP. Singlet oxygen is directly monitored by measuring its phosphorescence emission at ∼1280 nm, with results shown in Fig. 3a. Note that in these measurements AgNP@BCP@HP has the same HP concentration as the pristine HP, and the same AgNP concentration as AgNP@BCP. Upon excitation at 395 nm, AgNP@BCP generates negligible singlet oxygen. However, the presence of AgNP@BCP greatly promotes the singlet oxygen generation by the entrapped HP, as compared to the pristine HP. The integrated areas from 1220 to 1340 nm for AgNP@BCP@HP and for pristine HP are calculated, respectively, and the enhancement is determined to be ∼3.

Fig. 3 Singlet oxygen emission (a) and excitation spectra (b) of AgNP@BCP, pristine HP and AgNP@BCP@HP. [HP] is 10 μM and [AgNPs] is 0.17 mg mL⁻¹.

The interaction between the entrapped HP and AgNP@BCP simultaneously leads to change in the excitation profile of HP for singlet oxygen generation. As shown in the singlet oxygen phosphorescence excitation spectra (Fig. 3b), while pristine HP can only be excited at ∼395 nm to produce singlet oxygen, AgNP@BCP@HP displays a broadened excitation profile, extending into the red/NIR region. This feature of the hybrid photosensitizers has important implication in the PDT and PDI applications.

AgNP@BCP@HP demonstrates very high efficacy in the photoinactivation of both Gram-negative and Gram-positive bacteria under white light illumination. In the PDI tests, Staphylococcus epidermidis (ATCC 35984) is used as the model Gram-positive strain and Escherichia coli (ATCC 35218) as the model Gram-negative strain, and a non-coherent lamp is used as the light source for illumination. Detailed data of the PDI tests are included in the ESI.† Results are summarized in Fig. 4, where the enhancement in PDI efficacy is defined as log₁₀(enhancement killing) = log₁₀(AgNP@BCP@HP killing) − log₁₀(pristine HP killing) − log₁₀(AgNPs killing). This enhancement represents the promoter effect of AgNP@BCP on the entrapped HP in the PDI efficacy. As shown in Fig. 4a, there is a marked difference in the PDI efficacy against S. epidermidis among AgNP@BCP, pristine HP and AgNP@BCP@HP. AgNP@BCP displays only a little bacterial killing, while pristine HP has moderate PDI efficacies under white light illumination. In contrast, AgNP@BCP@HP displays much higher PDI efficiencies than the sum of AgNP@BCP and HP, up to >5 orders of magnitude. Similar effect is also observed in the PDI tests against the Gram-negative E. coli, with the enhancement in PDI efficacy up to ∼5 orders of magnitude (Fig. 4b).

Fig. 4 Killing efficacy of AgNP@BCP@HP, pristine HP and AgNP@BCP against (a) S. epidermidis (ATCC 35984) and (b) E. coli (ATCC 35218) under white light illumination. Killing efficacy is shown as log₁₀(CFU mL⁻¹) vs. [HP]. Light fluence was 49 J cm⁻² in (a) and 294 J cm⁻² in (b).

We note that AgNP@BCP@HP can also photoinactivate bacteria under the red/NIR illumination due to their broadened excitation profile. In these tests, a bandpass filter is used to select the red/NIR portion of the lamp output to illuminate samples. Summarized results are shown in Fig. 5. Again, AgNP@BCP@HP displays much higher PDI efficacy against both Gram-positive (Fig. 5a) and Gram-negative (Fig. 5b) bacteria than the sum of efficacy of AgNP@BCP and HP acting separately. These results are in line with the enhanced singlet oxygen generation under broadened excitation shown in Fig. 3. One unusual observation in these red/NIR illumination experiments is that the killing efficacy of AgNP@BCP@HP against E. coli appears to be higher than that under white light illumination. We attribute this to the longer illumination time (20 minutes), as compared to that under white light illumination (12 minutes), which may incur photothermal effect on the bacteria. This possibility is indirectly supported by the results of the control runs involving only AgNP@BCP, where longer illumination time also leads to higher killing efficacy.

Fig. 5 Killing efficacy of AgNP@BCP@HP, pristine HP and AgNP@BCP against (a) S. epidermidis (ATCC 35984) and (b) E. coli (ATCC 35218) under illumination of red/NIR light. Killing efficacy is illustrated as log₁₀(CFU mL⁻¹) vs. [HP]. Light fluence was 139 J cm⁻² in (a) and 277 J cm⁻² in (b).

Furthermore, we have assessed the cytotoxicity of the hybrid photosensitizers. In these experiments, viability of Hela cells was evaluated through the commonly used MTT assay, as described in the ESI.† Results shown in Fig. 6 indicate that the hybrid photosensitizers of concentrations used in the PDI tests display little toxicity under ambient light condition.

Fig. 6 Hela cells viability after incubated for 24 hours with AgNP@BCP@HP solutions containing different HP concentrations and with the corresponding AgNP@BCP solutions, respectively.

4. Conclusions

In summary, we note several unique features of the hybrid photosensitizer based on amphiphilic block copolymer stabilized silver nanoparticles: (1) the hybrids display very high PDI efficacy against both Gram-positive and Gram-negative bacteria. (2) The hybrids can function under both white and red/NIR light illuminations, the latter of importance to applications involving tissue penetration. (3) The hybrids have little cytotoxicity under ambient light condition at concentrations capable of photodynamically inactivating bacteria. (4) The Ag@BCP platform provides a facile means to incorporate photosensitizing molecules with poor water solubility for use in aqueous media. These results demonstrate great potential of such hybrid photosensitizers and pave way for PDI and PDT applications.

Acknowledgements

N. A. acknowledges the donors of the American Chemical Society Petroleum Research Fund (51850-DN17) for support of this research. P. Z. acknowledges partial support from DoD Award DM102420 (W81XWH-11-2-0103).

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Footnote

† Electronic supplementary information (ESI) available: Scheme of polymer synthesis; proton NMR, UV-vis absorption and fluorescence spectra; photodynamic inactivation assay results. See DOI: 10.1039/c6ra01660j

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