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Abstract
The research and development of non-toxic, broad-spectrum and environmentally friendly ultraviolet absorbers remains no significant progress in recent years. We found that the ultraviolet absorption spectra can be regulated through modification of functional groups on carbon dots surface, and the modified carbon dots exhibiting good stability and functions of sunscreen (Sun protection actor reaches to 22) and anti-aging properties were experimentally demonstrated. Moreover, we figured out the ultraviolet absorption mechanism of carbon dots for the first time and confirmed the existence of non-fluorescent radiation energy traps. Carbon dots are expected to be widely used and commercialized as ultraviolet absorbers.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultraviolet (UV) radiation, not merely causes the skin dark complexion, erythema on the human skin, more seriously, it initiates melanoma and DNA damage so that skin cancer is induced [1]. According to the world health organization report, there are more and more non-melanoma and melanoma skin cancers due to exposure to UV radiation [2]. Another huge ham of UV radiation includes crack, decomposition and discoloration of natural, especially synthetic polymeric materials when exposed to UV radiation so that the service life of polymers decreases seriously [3,4], which led to 21-28 billion dollars loss of chemical products and increased by 6.5% year by year from the report of research institute Markets and Markets in 2016.
Indeed, UV absorber is very effective to inhibit UV radiation [5,6]. There are single-functional, broad-spectrum chemical UV absorbers and nano TiO2 and ZnO usually used in sunscreen cosmetics [7,8]. The more diverse UV absorbers are used in polymers, such as benzotriazoles, benzophenones, triazine UV absorbers and salicylic acid series, etc [3,5,9–11]. However, actually, most of chemical absorbers used in sunscreen impose more or less toxicity on skin [12,13], and there exists drawbacks of narrow absorption range, easy migration and no universal UV absorbers used in polymers owing to the dissimilarity of photosensitivity among polymers materials [14]. Meanwhile, it appears low anti-UV efficiency, weak dispersity and poor transmittance for physical absorbers, and even phototoxic for TiO2 [15,16]. Thus, It is urgent and significant to research and develop one-step synthesized novel UV absorbers with broad-spectrum UV absorption, prominent absorption efficiency, good stability and environmental friendliness.
In recent years, quantum dots (QDs) as UV absorber have attracted a wide attention due to its outstanding performance of UV absorption and visible transmittance rate owing to that of particle size of less than10 nm, which are expected to be revolutionary of traditional UV absorber. In 2007, ZnO QDs were firstly reported as UV-shielding agent that exhibited prominent UV-shielding properties and the potential application in the development of transparent UV-protective materials [17]. Then, the researches of ZnO QDs doped into polydimethylsiloxane, Sn and Ag modified ZnO QDs, even SnO2, SnS2 QDs were carried out to prevent UV radiation [18–20], but all of these above QDs present a very low UV absorption efficiency and narrow-band absorption so that these QDs usually have to be used together with nano TiO2 and organic UV absorbers.
Interestingly, graphene and carbon quantum dots (CQDs) in the past two years have been reported [21–23], all of which exhibited outstanding anti-UV application results and good compatibility in polymers. Especially for CQDs, it has been applied in packaging such as polyethylene terephthalate (PET) to reduce lose of nutrition. In addition. Due to the simple synthesis methods and low cost, CQDs were thought to be the most promising QDs as alternative of commercial UV absorbers compared with GQDs. However, the as-reported CQDs exist the drawbacks of narrow-band absorption and low UV absorption efficiency even if above 95% visible light transmittance, which limits the application of CQDs in high-transparency products such as packaging bag, agricultural film, etc.. Regrettably, in spite of that CQDs show promising application prospect, by now no further researches on the regulation of absorption spectrum, stabilities under various pH values, temperatures and UV radiation simulating practical environments, dispersity in different solvents, and no further applications as UV absorber of CQDs are carried out, besides, the UV absorption mechanism of QDs is still unclear.
Herein, we synthesized full-band UV absorbent CQDs by regulating the functional groups on CQDs surface, and demonstrated the outstanding stability of UV absorption efficiency of the as-prepared CQDs. To confirm the potential applications of CQDs as UV absorber in anti-aging polymers and skin sunscreen, we fabricated CQDs and polyvinyl alcohol (PVA) composite films and emulsion with strong fluorescence under UV radiation using a simple dissolution-curing method, then evaluated experimentally and exhaustively. Finally, we proposed the UV absorption mechanism that is related to conjugated structure existing inner CQDs particles. Hence, the functionalized CQDs achieving full-band UV absorption can be used as a promising UV absorber, and the proposed mechanism helps us understand process of energy transfer of UV radiation absorbed by CQDs.
2. Experimental section
Synthesis of materials
Synthesis of surface groups modified CQDs: Three typical of CQDs with different UV absorption spectra and absorption efficiency were synthesized by hydrothermal treatment of citric acid (CA) and N source. Synthesis of the first type of CQDs (T1-CQDs): CA (1.21 g) and Urea (1.60 g) were dissolved in deionized water (28 ml), the mixed solution was then transferred into a polytetrafluoroethylene hydrothermal reactor (40 ml) and heated at 200 °C for 5 h. Synthesis of the second type of CQDs (T2-CQDs): the same as the T1-CQDs synthesis route except that glycine instead of urea. Synthesis of the third type of CQDs (T3-CQDs): CA (1.21 g) were dissolved in deionized water (20 ml), Ethylene glycol (8 ml), N,N'-bis(2-aminoethyl)-1,3-propanediamine (1.6 ml) were added in turn, then the mixed solution were transferred into a polytetrafluoroethylene hydrothermal reactor (40 ml) and heated at 200 °C for 5 h. The reaction solution were cooled to room temperature naturally, and filtered (using water phase needle filter with hole diameter of 0.22 µm), dialyzed for 24 h (using Dialysis bag with a molecular weight of 1000), suspension steamed at 70 °C under vacuum to obtain viscous liquid T1-CQDs, T2-CQDs, T3-CQDs, respectively, some of these were freeze-dried to obtain solid state T1-CQDs, T2-CQDs, T3-CQDs. Preparation of T3-CQDs@PVA composites: PVA (molecular weight of 27000-32000) (5 g) was dissolved in deionized water (50 ml), the solution was stirred for 1.5 h under 60 °C, and added 0, 4.75, 9.50 14.25, 19.00, 23.75, 28.50, 33.25, 38.00, 42.75, 47.50, 52.25, 57.00, 61.75, 66.50 and 71.25 mg T3-CQDs, respectively, the PVA and T3-CQDs@PVA emulsion were got, and then transferred the glass dish (coating amount: 0.048 g/cm2), dried at 50 °C for 6 h, finally, the corresponding numbered T3-CQDs@(PVA1-15) films with mass fraction of appropriately 0.1-1.5% at interval of 0.1% were obtained.
Characterization methods
The UV-Vis absorption and transmittance spectra were recorded by an ultraviolet-visible spectrofluorometer (UV-2550, Shimadzu). Absolute radiation spectra of sunlight and monochromatic 365 nm light sources were recorded by optical fiber spectrometer (Flame. Ocean Optics, America). The thickness of films were measured by vernier caliper, and infrared imaging of PVA and T3-CQDs@PVA films were recorded by infrared imager (R500EXPro, AVIO, Japan). The FT-IR spectra of CQDs and composite films were taken on a Nicolet Avatar 360 FT-IR spectrophotometer. The X-ray diffraction (XRD) patterns were collected using a persee XD-2X/M4600. The high resolution transmission electron microscopy (HRTEM) images were taken in a JEOL-2010 electron microscope. Scanning electron microscopy (SEM) images were carried out using a XL-30-ESEM (FEI). Fluorescence spectra and time-dependent intensity curve were recorded with a fluorescence spectrofluorometer (F-7000, Hitachi). Human sun protection experiment was carried out by solar light model SPF601-300 with xenon arc light source (Solar Light Co., Glenside, PA). (ESR) were recorded by electron spin resonance instrument (JES FA200, JEOL, Japan). Zeta value and curves were measured by zetasizer nano ZS (Malvern). Tensile tests were taken on a universal testing machine (UTM-4204, Ttzh, China), the loading rate was 5 cm·min−1 and the specimens had a width of 10 mm and at thickness of 0.03 mm.
UV transmittance and absorption rate calculation
UV transmittance spectra of T3-CQDs@PVA and PVA films were performed by ultraviolet-visible spectrofluorometer using PVA blank film as a reference, then UVB blocking rate (BUVB), UV transmittance (TUV) and absorption rate (Ab) were calculated according to Eqs. (1), (2) and (3).
where Q(λ), Q0(λ) on behalf of UV transmittance spectra of T3-CQDs@PVA and PVA films, S0, S1 is spectral integral area of Q0(λ) and Q(λ) at range of 280-400nm, respectively. A0, A1 is spectral integral area of Q0(λ) and Q(λ) at range of 280-320nm,respectively.
3. Results and discussion
Schematic spectrograms of UV absorption
Three typical CQDs (T1-CQDs, T2-CQDs, T3-CQDs) with different functional groups on surface due to the introduction of different water-soluble N-source chemicals were synthesized via the bottom-up approach, and their schematic diagram of functional groups on surface was illustrated as shown in Fig. 1(a). The UV-Vis absorption spectra shown in Fig. 1(b) vary obviously with the introduction of different functional groups on the CQDs surface. When the surface of the T1-CQDs is introduced -CO-NH- groups except the foundational groups of (-OH, -COOH), the synthesized CQDs have a UV absorption band with a band gap of 3.44 eV to 4.13 eV presented in Fig. 1(b) and 1(c) marked in green lines. When -COOH is replaced by -CO-NH-COOH, the absorbent-spectrum band that T2-CQDs correspond to occurs blue-shift wholly so as to UVB absorption band is formed presented in Fig. 1(b) and 1(c) marked in blue lines. Whereas, the absorption band gap of the T3-CQDs amplifies significantly when an amount of –NH- instead of -CO-NH and introduction of –OH in manner of –COOCH2- on the CQDs surface which forms a spectral absorption band of 3.10 eV to 4.43 eV containing both UVA and UVB presented in Fig. 1(b) and 1(c) marked in red lines. All these different absorption bands are attributed to orbital electrons transition of n &π to π* mainly induced by different functional groups, and T3-CQDs present full-band UV absorption that is significantly superior to the as-reported CDs which exhibited narrow-band absorption with half-width of appropriately 40nm from Hess et al.. Importantly, T3-CQDs also present high UV absorption rate as shown in Fig. 1(e), when the mass fraction of T3-CQDs in PVA film is 0.7%, the absorption rate reaches to 99%, and with the increase of doping concentration, the absorption rate gradually increase so that up to 100%. Besides, compared with the reported CDs synthesized by CA and polyethylene imine, T3-CQDs exhibits more than twice of the UV absorption intensity [22].
Fig. 1 Schematic diagram of functional groups (a), absorption spectra (b), transition energy level diagrams (c), FT-IR spectra (d) of three typical of CQDs (T1-CQDs, T2-CQDs, T3-CQDs). Absorption rate of various concentration of T3-CQDs (e), HRTEM images (inset: lattice image) (f), XRD pattern (g) of T3-CQDs, and Zeta potentials (h) of T3-CQDs in water, DMF and PVA aqueous solution.
To clarify the effects of chemical groups on CQDs surface on the UV absorption properties, the measurement of FT-IR spectra shown in Fig. 1(d) was carried out. Obviously, the functional groups on T1-CQDs, T2-CQDs, T3-CQDs surface contain –OH (peak at 1678 cm−1) and –C = O (peak at 3415 cm−1) characteristic groups because of that citric acid as the same carbon source. The peak at 1581 cm−1 is associated with stretching vibrations of –NH, and its absorption intensity the T3-CQDs corresponds to is much higher than T1-CQDs and T2-CQDs, which demonstrate that N,N'-bis(2-aminoethyl)-1,3-propanediamine is more conducive to –NH groups modified on the surface CQDs than glycine and urea and induce a full UV absorption band.
The HRTEM images and zeta potential were measured to assess the particle size, morphology, and the dispersion of T3-CQDs aimed at obtaining high visible light transmittance and transparency of products. The HRTEM images in Fig. 1(f) show that the lattice stripe radius is 0.21nm which agree with that of (100) planes as shown in the X-ray diffraction (XRD) shown in Fig. 1(g) pattern within the graphite, and the average particle size of T3-CQDs with a regular sphere the same as major reported other CQDs is only 5-6 nm, so, the T3-CQDs is particularly suitable for materials that require high transparency, low haze, and high clarity values, such as agricultural films, packaging films, and medical films [5].
In addition, zeta potential datas in Fig. 1(h) show that the absolute values of the zeta potentials present gradual increasing trend with the enhancement of CQDs doping concentration in water, N,N-dimethylformamide (DMF) and PVA aqueous solution, which gives further proof for good dispersion of CQDs in case of particle agglomeration that causes a decrease in UV absorption efficiency and yellowing. In short, the as-prepared T3-CQDs present broad-spectrum UV absorption properties which exhibit the applying potential as a series of novel UV absorbers.
UV absorption efficiency and stability
For simplicity, we prepared T3-CQDs@PVA films and emulsion with different T3-CQDs doping concentrations, and tested the transmittance spectra under solar radiation. The results as shown in Fig. 2(a) present the TUV value of T3-CQDs@PVA film (thickness 0.03 mm) decreases to almost 0% for the transmittance of UVA (320~400 nm) and less than 2% for the transmittance of UVB (280~320 nm) when the concentration increase to 0.64 μl/g from 0.45 μl/g, which meets the spectral requirement of broad-spectrum UV absorbers. Meanwhile, T3-CQDs@PVA films show prominent visible light transmittance that is nearly the same as pure PVA fillm, and which is consistent with GQDs@gel glass and CQDs-PVA coating on PET reported by Xie and Hess, respectively [22,23].
Fig. 2 UV absorption efficiency and stability of T3-CQDs@PVA. (a) Transmittance spectra of T3-CQDs@PVA films versus PVA film at the same thickness of 0.03mm (inset: Photograph of Rounded T3-CQDs @PVA7). Absolute radiation spectra of PVA and T3-CQDs@PVA7 films under solar light (b) and under UV365 monochromatic light (c). (d) Logarithmic analysis of T/T0 (T0, T represent TUV value of p-CDs@PVA(5,6,7) films under 25°C and other temperature, respectively, and the films were tested after kept for 1h). (e) Logarithmic analysis of T/T0 (T0, T represent TUV value of p-CDs@PVA(5,6,7) films under pH equal to 8 and other pH value, respectively, which were obtained by pH adjustment of p-CDs @PVA aqueous solution and then drying for 30 min at 55 °C). (f) Logarithmic analysis of T/T0 (T0, T represent TUV value of p-CDs@PVA (5,6,7) films before and after aging exposed to UV340 lamp.
A circular T3-CQDs@PVA7 film of a thickness of 0.03 mm and a diameter of 9 cm is fabricated to assess visible-light transmission as shown in inset of Fig. 2(a). The film presents good transparency and non-hazy which indicate that the absence of CQDs agglomerates. Meanwhile, the corresponding excellent UV absorption performance of the film is revealed as shown in Fig. 2(a).
Then, we have researched on the variation law of the absolute radiation intensity of PVA film and T3-CQDs@PVA films under solar radiation at noon which is presented in Fig. 2(b). Interestingly, there are no differences in solar radiation intensity between under PVA film and no film, and the solar radiation intensity less than 400 nm markedly decreases to approximately 1.3% for the sample of T3-CQDs@PVA7 film. Meanwhile, in the visible-light region, the solar radiation energy stays stable. To verify whether the same absorption efficiency of T3-CQDs@PVA7 film is under higher-intensity UV radiation, we used monochromatic UV365 light with UV radiation intensity about 13 times than UV radiation in solar radiation to detect the transmittance, the spectra in Fig. 2(c) show that the absorption rate of UV radiation energy for the same T3-CQDs@PVA7 film is only about 1.5% lower than under solar radiation, which indicates that T3-CQDs have distinct and stable UV absorption efficiency with tiny impact led by radiant energy. However, actually, many commercialized UV protection products have great discounts on sun protection and anti-aging effects under higher-intensity UV radiation, which imply that T3-CQDs with outstanding and stable UV absorption ability can be applied in a wider range than some commercial UV absorbers [24,25].
To achieve a practical purpose, we have studied the UV absorption properties and stability of T3-CQDs @PVA films and emulsion under varying temperature conditions, pH and aging time with UV340 lamp as radiation light source shown in Fig. 2(d)-2(f). Figure 2(d) presents the variation of TUV value under solar radiation as the temperature rising from −18 °C to 75 °C. When the temperature is below 0 °C, the TUV value decreases significantly compared to room temperature conditions, which possibly is assigned to ice turned by trace amount of water existing in T3-CQDs@PVA(5,6,7) films that is conducive to UV absorption by CQDs because of the refractive index of ice (n = 1.3090) is slightly lower than that of water (n = 1.3333). Whereas, the TUV value of T3-CQDs@PVA(5,6,7) films under the temperature from 0 °C to 75 °C keep stable. Subject to the low embrittlement temperature of PVA, T3-CQDs@PU (polyurethane) films resistant to high temperature were fabricated to study UV transmission performance of T3-CQDs at higher temperature, and the TUV show that the UV absorption spectra are basically constant even though the temperature up to 150 °C or more higher if better temperature resistant polymer used as matrix. Therefore, CQDs can be applied in a majority of practical applications environment in our daily production activities basing on its outstanding temperature resistance.
Acid and alkali resistance of UV absorbers determines the application range and even effects the productive-processing technique of chemical products. For example, UV absorber used in outdoor coating and open-air polymer products must have acid resistance to prevent its decomposition or decline of UV absorption efficiency caused by acid rain. Based on the problem, we have studied the UV absorption properties of T3-CQDs@PVA aqueous solution under varying pH value shown in Fig. 2(e). The TUV value of T3-CQDs@PVA aqueous solution shows distinct changes with varying pH values. In detail, the UV absorption capacity gradually decreases when the pH value drops to below 4, we think this is caused by disappearance of fluorescent radiation transitions because of that CQDs are fluorescently quenched under strong acid, but there is merely 1%-2% decline for TUV transmittance. When the pH value increases from 5 to 14, the TUV value of T3-CQDs@PVA aqueous solution is basically constant. Therefore, T3-CQDs are suitable for various extreme conditions of acidity and alkalinity because of its extremely distinct UV absorption stability, and even can be added during the synthesis process of polymer materials that is not available for commercialized UV absorbers.
Figure 2(f) presents the TUV value of T3-CQDs@PVA (5,6,7) films gradually increase with the extension of UV continuous radiation time that means the UV absorption capacity of T3-CQDs is gradually weakening. We think that the underlying reason is that the molecular bond energies of N-C, C-O, and N-H on CQDs surface are 291.6 KJ/mol, 351.5 KJ/mol, and 390.8 KJ/mol, respectively, all of which are lower than the UV radiation energy in the solar light. Although most of the UV radiation energy is absorbed and reflected by the CQDs, a small amount dose of UV radiation can still break these molecular bonds and result in the destruction of the transition energy levels so that the decrease of UV absorption efficiency [26]. However, after 600 h accelerated UV aging test (main wavelength: 340nm; power: 800w), the TUV value only decrease from 93.0% to 88.3% for T3-CQDs@PVA5 film, which means good photostability for the T3-CQDs doped in PVA. Contrastively, after 72 h accelerated aging for T3-CQDs@PU with the same conditions as T3-CQDs@PVA, the TUV transmittance decays to 100% that lose UV absorption performance. Therefore, the photostability of T3-CQDs is related to the matrix, and the detailed mechanism needs to be further research.
Consequently, T3-CQDs@PVA composites present UV broad-spectrum absorption and outstanding UV absorption efficiency under solar and stronger UV radiation while maintaining high visible-light transmittance. Meanwhile, outstanding resistance to high temperature, acid and alkali, and UV radiation of T3-CQDs in PVA matrix show enormous potential application in anti-UV chemical products.
Mechanism of UV absorption
Combining the UV absorption properties of T3-CQDs as shown in Fig. 2, we figured out the mechanism diagram of UV absorption properties of CQDs shown in Fig. 3, obviously, the UV broadband absorption of CQDs should be attributed to the n→π* and π→π* electronic transitions.
Fig. 3 (a) The comparison of UV absorption spectra between IPCA and CQDs dispersed into 5 ml water. (b) Fluorescence spectra of the external solution of dialysis with various time of dialysis (inset: photograph after 18 h dialysis). (c) Fluorescence spectra and corresponding UV transmittance of p-CD aqueous solution when pH vary from 7 to 1, and T3-CQDs after 250 °C treatment for 2 h. (d) Infrared images of PVA films with various doping concentrations of CQDs.
Due to the dimensionless scale number a = 2πr/λ is far much less than 0.1 within the range of 200-400 nm, this meets rayleigh scattering which facilitates the UV absorption of CQDs. Obviously, this is not fundamental reason because of that other QDs such as ZnO with weak and narrow-band absorption [17]. The large conjugated structures exist in fluorophores (IPCA) formed at the range of 140-150 °C which are the typical of UV absorbent characteristic group, even though IPCA are converted to CQDs when pure IPCA underwent the hydrothermal process (200 °C) as reported by yang et al. [27]. To further check the signal origin of UV absorption, we carried out the comparison of UV absorption spectra between IPCA and CQDs, fluorescence test on the external solution of dialysis with various time of dialysis as shown in Fig. 3(a) and 3(b), respectively. At the same concentration, the UV absorbent intensity of CQDs is more than 10 times than IPCA, and there is a wider UV absorption band for CQDs, which indicates that IPCA are nearly converted to CQDs also with conjugated structures inner CQDs particles. In addition, fluorescence intensity of the external solution of dialysis present gradual decrease with extending time of dialysis so that there is no fluorescence after 18 h of dialysis, and the inset photograph also demonstrate that the small molecule fluorophores are eliminated. Thus, we think that the signal of UV absorption origins from CQDs and there mainly includes two routes of UV absorption as follows.
The first route is that electron transfers to the excited states and emit strong blue light along with the release of weak energy. To confirm this, we weakened the fluorescence to quench by adjusting the pH value and through temperature treatment at 250 °C for 2 h shown in Fig. 3(c) (up), however, the UV absorption efficiency has only slightly decline according to the Fig. 3(c) (bottom), which demonstrate that there is indeed a positive correlation between fluorescence luminescence efficiency and UV absorption efficiency, but it is not fundamental reason. The second UV absorption route is that most of UV energy is absorbed by conjugated structures of CQDs. For the further confirmation, we tested the infrared imaging of PVA and T3-CQDs@PVA films with different T3-CQDs doping concentration shown in Fig. 3(d). The surface temperature of T3-CQDs@PVA films increases under UV irradiation as the concentration of T3-CQDs enhances, and the temperature under T3-CQDs@PVA7 film increase about 2.3 °C compared to PVA film, this explains that energy traps in T3-CQDs really exist and CQDs absorb most of the UV radiation energy and release in manner of heat energy.
In short, rayleigh scattering and fluorescence luminescence efficiency indeed facilitate the UV absorption, but the foundational signal of UV absorption origins from CQDs with conjugated structures. The corresponding UV absorption mechanism of CQDs is illustrated as shown in Fig. 4.
Fig. 4 The UV absorption mechanism illustration of CQDs. (Left) Presents the UV absorption route of CQDs when fluorescence is quenched induced by pH regulation. (Middle) The UV absorption of CQDs includes two routes, route I is that UV energy transfers to the excited state and emits fluorescence along with heat radiation, route II is that UV energy is absorbed by CQDs with conjugated structures, and converges into the energy trap and releases it as heat energy. (Right) The typical conjugated structure of CQDs.
Sunscreen and anti-aging properties
Firstly, to test whether the T3-CQDs has sun protection function, we carried out a human body sunscreen experiments according to the industry-standard conditions using the samples of PVA and T3-CQDs@PVA emulsion . At the condition of 1 minimal erythema dose (MED) radiation, the number of erythema appearing on the skin of tester coating PVA emulsion increases visibly with the enhancement of radiation intensity (197.00, 246.25, 307.81, 384.77, 480.96, 601.20 μw/cm2 corresponds to location 1, 2, 3, 4, 5, 6 respectively) which is presented in Fig. 5(a) (right, up). However, there is no erythema appearing for coating T3-CQDs@PVA emulsion at 5 MED radiation dose shown in Fig. 5(a) (right, down). In addition, according to the industry coating standard of 2 mg/cm2, we prepared a T3-CQDs@PVA film, and its UV transmittance spectrum shows that it has not only a strong UVA absorption, but also a significant UVB absorption efficiency which are presented in Fig. 5(b). The BUVB value reaches to approximately 95.1% calculated theoretically, and the corresponding SPF is about 22 according to the reference curve between UVB absorption rate and SPF. Therefore, the modified T3-CQDs can be applied in sunscreen as a broad-spectrum UV absorber to prevent human from erythema and other skin diseases induced by UV radiation.
Fig. 5 Application of CQDs as UV absorbers for sunscreen product. (a) Human sunscreen experiments based on whether erythema appears and the number of appearances under UV radiation within 280-320 nm through coating PVA and T3-CQDs@PVA7 emulsion on the body back with the coating weight of 2 mg/cm2, and the radiation dose of location 1, 2, 3, 4, 5, 6 is 197, 246.25, 307.81, 384.77, 480.96, 601.20 μw/cm2, respectively. (b) SPF value calculated based on transmittance spectrum of T3-CQDs@PVA7 film under the radiation of solar light, meanwhile, visible light above 400nm remains above 90%.
Secondly, to verify the anti-aging effects in the polymer, PVA and T3-CQDs@PVA5 films were prepared and the accelerated aging test was carried out under radiation of UV340 lamp with power of 800W. The surface groups, morphology and mechanical properties of the two films were characterized at the aging time of 0 h and 430 h. The SEM images presented in Fig. 6(a)-6(c) show the surface morphology of the PVA and T3-CQDs@PVA7 films before and after aging. Initially, there are smooth surface for both the PVA and the T3-CQDs@PVA7 films as shown in Fig. 6(a). As the UV radiation time prolongs, the molecular chains of the PVA surface are partially broken, and a large amount of particles were produced by the crosslinking of the molecular chains escaping until a deeper fault appears that can be seen clearly in Fig. 6(c). However, contrastively, the T3-CQDs@PVA7 surface shows only a point-to-surface protrusion shown in Fig. 6(b), which manifests that T3-CQDs indeed improve anti-aging properties of PVA and prolong its service time due to UV absorption characteristic of T3-CQDs.
Fig. 6 Application of CQDs as UV absorbers for anti-aging of polymers. (a), (b), (c) SEM images of PVA or T3-CQDs@PVA7 film (photo-aging for 0 h), T3-CQDs@PVA7 films (photo-aging for430 h) and PVA film (photo-aging for430 h), respectively. (d), (e) The corresponding stress-strain curves and FT-IR spectra of PVA and T3-CQDs@PVA7 films before and after UV340 photo-aging, respectively.
Furthermore, the stress-strain behaviors of PVA and T3-CQDs@PVA7 films before and after UV340 aging were examined presented in Fig. 6(d). Interestingly, T3-CQDs shows a outstanding elasticity-improved effect. The patterns in Fig. 6(d) present the initial tensile strength and elongation at break of PVA film were 50 Mpa and 28% respectively, but the tensile strength of T3-CQDs@PVA7 film decrease to 26 Mpa and the elongation at break increase significantly by 471% probably due to the changes of orientation degree of PVA molecular chain led by the van der Waals forces between T3-CQDs and PVA. However, Mechanical properties show great different effects led by CQDs when the matrix changes. For example, for the matrix of polar hydrophilic carboxylated acrylonitrile butadiene rubber, only the stress performance is improved after blending with CQDs [28]. If the surface of the CQDs has more or longer chains of organic groups than T3-CQDs, such as aspirin-based CQDs with good biocompatibility, it probably shows better improvement of mechanical properties of polymers [29]. Importantly, the tensile strength of PVA film decrease seriously after continuous 430 h UV radiation, whereas, the tensile strength and elongation at break of T3-CQDs@PVA7 film decrease slightly just at a smaller amplitude, which demonstrates that the T3-CQDs can be used to protect the molecular chain of PVA from UV radiation the same as commercialized UV absorbers. FT-IR patterns shown in Fig. 6(e) illustrate the variations of chemical groups at 0 h and 430 h aging time. There are 4 typical absorption peaks at around 1140, 1250, 1750 and 2950 cm−1, which are attributed to the stretching vibration of C-C, stretching of C-O vibration, flexural vibration of O-H, asymmetric stretching vibration of H-C-H, respectively. The absorption peak at 3350 cm−1 is attributed to the stretching vibration of the O-H bond from the water. Initially, no T3-CQDs were detected due to only about 0.64 µl/g T3-CQDs concentration in the T3-CQDs@PVA7 film so that the CQDs overlap with the FT-IR of the PVA film. Apparently, no changes of the characteristic absorption peak were found in the FTIR spectra of T3-CQDs@PVA7 film after accelerated aging for 430 h which indicates T3-CQDs can indeed protect the chemical groups on PVA by absorbing UV light. However, the intensity of the absorption peak at 1140, 2950 and 3350 cm−1 decreased significantly after accelerated aging for 430 h for PVA film that means the C-C bond and H-C-H bond were broken and the water peak disappeared during the UV radiation, which are consistent with the results shown in the SEM image and give the further proof that T3-CQDs play a vital role in prolonging the lifetime of polymers owing to its UV absorption performance.
4. Conclusion
In this work, we have demonstrated that T3-CQDs present UV broad-spectrum absorption and high UV absorption efficiency and good stability, and confirmed that the unique capability of UV absorption range and absorption efficiency of T3-CQDs achieved through the modifying of functional groups on T3-CQDs surface. We have proposed the UV absorption mechanism of T3-CQDs that mainly includes fluorescent and thermal radiation transition’s absorption, and the thermal radiation transition’s absorption originated from conjugated structure plays the fundamental role. The experimental results of sunscreen and polymer anti-aging show the outstanding UV absorption properties to protect skin and significant prolonging of service life by preventing the breakage of polymer chains from UV radiation. Besides, T3-CQDs with other outstanding performances of one-step synthesis and non-toxic or low-toxic possibly be the potential revolutionary of commercialized UV absorbers.
Funding
National Natural Science Foundations of China (Grant Nos. 21571067, 51402207); Teamwork Projects funded by the Guangdong Natural Science Foundation (Grant No. S2013030012842).
Disclosures
The authors declare no competing financial interest.
References
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