Letter Cite This: ACS Appl. Mater. Interfaces 2017, 9, 44259−44263
www.acsami.org
Stable and Photoswitchable Carbon-Dot Liposome Tzu-Heng Chen† and Huan-Tsung Chang*,†,‡ †
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan Department of Chemistry, Chung Yuan Christian University, Taoyuan City, Taiwan
‡
S Supporting Information *
ABSTRACT: Carbon-dot (C-dot) liposome consisting of several thousands of C-dots shows interesting photoswitching properties. The water-dispersible C-dot liposome possesses intrinsic photoluminescence (PL) and is stable against salt and photoirradiation. The PL of C-dot liposome can be turned off and then on under photoirradiation over the wavelength regions of 510−540 nm and 365−420 nm, respectively. Like reported C-dots, the C-dot liposome emits various colors when excited at different wavelengths. Having great stability and high contrast, images of individual C-dot liposome have been recorded, showing negligible photoblinking. Through a simple photolithographic approach, micropatterns of C-dot liposomes emitting different colors have been fabricated.
KEYWORDS: C-dot liposome, photoswitching, multiple emission, high contrast, micropatterns
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Interestingly, the C-dot liposome is in its off state after it is excited at 530 nm and then transforms to on state upon photoirradiation at 405 nm. C-dot with lipid-like properties were first prepared from triolein through a simple heating and hydrolysis process. The C-dots were subjected to dialysis and then passed through a polycarbonate membrane filter having pore sizes of 100 nm to obtain C-dot liposomes. The hydrodynamic diameters of spherical C-dot liposomes were determined by dynamic light scattering (DLS) to be 93 ± 25 nm (Figure S1). The transmission electron microscopy (TEM) image (Figure 1A) displays that C-dot liposomes have vesicle structures with a mean diameter of 103 ± 15 nm (n = 20). The HRTEM image of one representative C-dot liposome shown in Figure S2 reveals that each C-dot liposome has an unilamellar bilayerstructure and consists of several ten thousands of small C-dots (mean size 1.9 nm), mainly through amphiphilic interactions among the surface ligands, including hydrophobic interactions between the oleate groups. It is important to point out that the C-dots prepared from small molecules such as glycine can not form vesicle structures, mainly because their surfaces are rich in hydrophilic functional groups such as hydroxyl, amino, and carboxylates.13−15 Figure 1B shows differential Raman spectral profiles of C-dot liposome and triolein (carbon source). In addition to the characteristic peaks at 1327, 1361, 1444, and 1593 cm−1 (marked with black stars) for triolein, peaks at 1407 and 1535 cm−1 (marked with red stars) that are assigned to the D and G bands are apparent in the C-dot liposome. The C-dot
hotoswitchable luminescent materials have potential in sensing and for fabrication of data storage devices.1 They can transform from a thermodynamically stable state to a metastable photostationary state upon photoexcitation.2 Through thermal relaxation or photoirradiation at a different wavelength, the molecules in the photostationary state return to equilibrium. The on (bright) and off (dark) states can process for numerous cycles.3 Various photoswitchable materials have been prepared and used for different applications such as cell imaging.4−10 Organic compounds such as azobenzene derivatives undergo photoconversion through changes in their chemical structures like cis−trans isoforms.4−7 Through photoswitchable Fö rster resonance energy transfer (FRET), dye-cross-linked dendritic nanoclusters and nanomaterials containing two different organic dyes have been developed.8,9 Polymer dots prepared from organic precursors are photoswitchable with high contrast.9−11 Fluorescent proteins are photoswitchable reversibly through cis−trans conversion of their chromophore moieties under visible light irradiation.12 Each of the reported photoswitchable materials has only one emissive color besides the one using two different dyes. In addition, weak photoluminescence (PL), photoblinking, low contrast, short-shelf lifetime, difficult preparation, poor water solubility/dispersibility, and/or high cost are sometimes problematic. In this study, photoswitchable C-dot liposome, with high contrast and negligible photoblinking, was prepared from triolein. Like most reported C- dots,13−15 the as-prepared C-dot liposome shows excitation-wavelength-dependence PL properties. The C-dot liposome in liquid solution and on the surface of glass both show interesting photoswitchable optical properties and emit different colors upon excitation at various wavelengths. © 2017 American Chemical Society
Received: October 3, 2017 Accepted: December 8, 2017 Published: December 8, 2017 44259
DOI: 10.1021/acsami.7b14969 ACS Appl. Mater. Interfaces 2017, 9, 44259−44263
Letter
ACS Applied Materials & Interfaces
Figure 2. Photoswitching of C-dot liposomes after they were excited separately at 405 and 530 nm. (a) PL images and (b) reversible photoswitching. (c) PL intensity trajectories show negligible blinking properties of C-dot liposomes after they were excited at 530 nm. The on/off of excitation shutter was used to control the light at 530 nm.
Figure 1. Characterization of C-dot liposomes through TEM, Raman light scattering, and PL measurements. (A) TEM image. Inset to A: Magnified TEM image shows that the C-dot liposomes have vesicle structures. (B) Raman spectra. The Raman spectrum of triolein (ii) is provided as a control. Laser emitting at 532 nm was used for the Raman analysis. (C) PL photographs. Excitation wavelengths are (i) 365, (ii) 405, (iii) 448, (iv) 488, and (v) 530 nm, respectively.
stronger light source is applied. The on/off PL intensity ratios of each C-dot liposome and individual C-dot are around 1140 and 20, revealing a great signal contrast of the C-dot liposome that has great potential for use in data storage.26,27 Such a great contrast is mainly due to the signal originating from many Cdots in each of the liposome and the nature (photostability and high PL) of each C-dot. The contrast value provided by single C-dot liposome is higher than that provided by the reported photoswitchable fluorescent materials.10,28−31 Figure 2C displays that the PL of single C-dot liposome was stable once the shutter was off (no further irradiation at 530 nm), the PL became weaker once the shutter was on. The smooth trajectories show negligible blinking characteristics of the Cdot liposome. Having advantages of extremely high photostability, brightness, stability against salt, and water dispersibility, the C-dot liposome holds great potential for sensing and cell imaging. The interesting photoswitching characteristics of C-dot liposome was further investigated using a laser scanning confocal microscope (LSCM) equipped with three lasers. One cycle of the excitation → deactivation → activation → ground state → excitation → emission process is summarized as shown in the path of i → ii → iii → iv → v (Figure 3). The C-dot liposomes were excited and then deactivated upon excitation at 530 nm. The deactivated C-dot liposomes (by 530 nm) were activated through photoirradiation with the laser at 405 nm. After the C-dot liposome returned to their ground state, they were excied separately with lasers at 458, 476, 488, and 496 nm, leading to green, chartreuse, amber, and vermillion PL images. We note that they were not excited efficiently at the four wavelengths from their off state directly. To gain more insight about the interesting photoswitching property of C-dot liposome, time-dependent PL images of Cdot liposomes adsorbed onto a PDDA treated indium tin oxide (ITO) glass under photoirradiation at 530 nm were recorded separately in the absence and presence of an applied voltage of 1.0 V (Figure S7). Electric field speeded up the PL decay of a representative single C-dot liposome in the anode (15 and 34 s were required separately to reach 90% PL intensity decreases in the absence and presence of electric field), whereas it suppressed the PL decay completely in the cathode. The result
liposomes in aqueous solution (Figure 1C) and on the surface of a poly(diallyl-dimethylammonium chloride) (PDDA) coated cover glass slide (Figure S3) both possess interesting excitationwavelength-dependent emission properties.13−24 The inset shows their emission colors (emission wavelengths) are cyanblue (482 nm), cyan (506 nm), amber (561 nm), and vermilion (612 nm) upon excitation at the wavelengths of 375, 400, 488, and 530 nm, respectively. Their PL intensity ratio is 10:5:1:1, revealing blue PL is the strongest one which is similar to most reported C-dots.13−21 Like most reported C-dots having multiple emissive states, Figure S4 shows a multiple exponential PL decay of the C-dot liposome, with three lifetimes of 6.2 ns (59%), 1.6 ns (33%), and 17.1 ns (8%).25 The C-dot liposomes are well-dispersed in aqueous solution, in which they are extremely stable against salt (up to 1 M NaCl) and are photostable (less than 5% change in their PL intensity) under irradiation with an Hg lamp (100 W) for 2 h. The C-dot liposome solution emitted red emission upon excitation at 530 nm, but its PL almost disappeared after photoirradiation for 20 min (Figure S5). Its blue PL restored after excitation at 405 nm for 1 min. The PL restoration was only achieved under photoirradiation over the wavelength region of 365−420 nm. The interesting photoswitching property was also observed on the C-dot liposome that was adsorbed onto a PDDA-coated cover glass slide through electrostatic interaction. After photoirradiation of the adsorbed C-dot liposome at 405 nm for 10 s, blue PL emitted from individual C-dot liposome appeared as shown in Figure 2A. The intensity trajectories of indicated particle was recorded in Figure S6. Upon irradiation at 405 nm, the blue PL gradually increased within 10 s. On the other hand, the red PL occurred immediately after irradiation at 530 nm. The red PL gradually decreased and finally diminished (about 40 s). Interestingly, Figure 2B displays that the on−off photoswitching of single Cdot liposome was fast (less than 10 s) and highly reversible (RSD of the PL intensity change between the first and the 40th cycle is
