DNA@Mn3(PO4)2 Nanoparticles Supported with Graphene Oxide as Photoelectrodes for Photoeletrocatalysis

A novel deoxyribose nucleic acid (DNA)-based photoelectrode consisting of DNA@Mn3(PO4)2 nanoparticles on graphene oxide (GO) sheets was successfully fabricated for photoelectrocatalysis. DNA served as a soft template to guide the nucleation and growth of Mn3(PO4)2 nanoparticles in the synthesis of Mn3(PO4)2 nanoparticles. More importantly, the DNA also serves as semiconductor materials to adjust charge transport. Under UV light irradiation (180–420 nm, 15 mW/cm2), the photocurrent density of DNA@ Mn3(PO4)2/GO electrodes reached 9 μA/cm2 at 0.7 V bias (vs. SCE). An applied bias photon-to-current efficiency (ABPE) of ~0.18% can be achieved, which was much higher than that of other control electrodes (<0.04%). In this DNA-based photoelectrode, well-matched energy levels can efficiently improve charge transfer and reduce the recombination of photogenerated electron-hole pairs. Electronic supplementary material The online version of this article (doi:10.1186/s11671-016-1784-z) contains supplementary material, which is available to authorized users.


Background
Energy crisis and environmental issue at global level are important topics and force us to search renewable clean energy [1]. Sunlight is an inexpensive, non-polluting, abundant, and endlessly renewable source of clean energy. Among the technologies for converting sunlight, photoelectrocatalysis combined the concepts of photovoltaics and wet-chemical photosynthesis has the ability to split water through solar energy to produce hydrogen [2]. Normally, broadly used photoelectrode materials are inorganic semiconductor material, such as metal oxides, silicates, and molybdates [3][4][5][6][7][8][9]. Phosphate can accelerate photoelectrochemical reactions and Li et al. reported cobalt phosphate as an oxygen evolution co-catalyst and used as a potential photoelectrocatalyst for efficient solar water splitting [10,11]. Manganese phosphate, (Mn 3 (PO 4 ) 2 , as one of phosphates has been used as cathode material for lithium ion battery [12], a water oxidation catalyst [13] and sensing materials [14]. As far as we know, no phosphate was studied as photoelectrocatalyst.
The method of solid-phase synthesis is commonly used to prepare Mn 3 (PO 4 ) 2 . Therefore, the size of synthesized Mn 3 (PO 4 ) 2 nanoparticles is in the range of several micrometers. Deoxyribose nucleic acid (DNA), a kind of nucleic acid carrying the genetic information of living organisms, shows to be promising in preparing materials with novel structures and functions [15]. Tremendous attention has been paid because of its efficiency and versatility in controlling of nanostructure growth. The programmable nature and rich molecular interaction with a variety of species enable DNA as a template to guide and support functional nanomaterials synthesis [16][17][18][19]. To reduce the size and increase the specific surface area, we use DNA molecules as a soft template for assisting crystal nucleation and uniformly growth of Mn 3 (PO 4 ) 2 . Moreover, as a p-type semiconductor, DNA can serve as partial electrode material for adjusting charge transportation [20][21][22].
Limited charge transport at the interface between nanomaterial and electrode surface is a critical issue in photoelectrocatalysis. Two-dimensional nanomaterial can provide the opportunity to functionalize this interface. As a star nanomaterial, graphene oxide (GO) was obtained by the oxidation of graphite powders and has a p-type semiconductor characteristics [8,23], resulting from oxygen's higher electronegativity than carbon atoms [24]. Besides, DNA and GO can bind together by strong interactions, such as van der Waals, π-π stacking, and combination of hydrogen [25].
Herein, we fabricated a novel DNA-based photoelectrode consisted with DNA@ Mn 3 (PO 4 ) 2 nanoparticles on GO sheets. The DNA as a soft template can assist the nucleation and growth of Mn 3 (PO 4 ) 2 nanoparticles. GO as the electron-blocking layer can reduce charge recombination, which can also enhance the light absorption partially. Under UV light irradiation (λ: 180-420 nm, 15 mW/cm 2 ), the photocurrent density of DNA@ Mn 3 (PO 4 ) 2 /GO electrodes can reach up to 9 μA/cm 2 at 0.7 V bias (vs. SCE). An ABPE efficiency of~0.18% can be achieved, which is much higher than that of other control electrodes (<0.04%). In this DNA-based photoelectrode, well-matched energy levels can efficiently improve charge transfer and reduce the recombination of photogenerated electron-hole pairs.

Synthesis of Graphite Oxide Nanosheets and DNA@Mn 3 (PO4) 2 Nanocomposite
Graphite Oxide (GO) Nanosheets Synthesis GO nanosheets were prepared from graphite powders by a modified Hummers method [26,27]. Firstly, 0.5 g graphite, 0.5 g NaNO 3 , and 23 mL H 2 SO 4 (98%) were together stirred in an ice bath, then slowly mixed with 3 g of KMnO4. The mixture was placed in a water bath at 35 ± 5°C stirring for about 1 h until the paste appears. Secondly, 100 ml DI H 2 O was added into the flask to dilute the mixture. Thirdly, 3 ml H 2 O 2 (30%) was dropcasted into the reaction mixture. Fourthly, product was harvested through centrifugation and thoroughly washed by HCl (5%, v/v) and DI H 2 O. GO (6 mg/mL) was dispersed use ultrasonic about 6 h with an ultrasonic power of 65 W. Finally, the homogeneous solution was centrifuged about 10 min to collect the supernatant for reserve.

DNA@Mn 3 (PO 4 ) 2 Nanocomposite Synthesis
The DNA@Mn 3 (PO 4 ) 2 was prepared according to literature [14]. In brief, firstly, 0.1 ml DNA (10 mg/ml) was dissolved into 0.9 ml DI H 2 O and boiled for 10 min then ice cooling to prevent forming of double strand helix. Secondly, 1 ml DNA solution, 1 ml MnSO 4 (0.1 M), and 9 ml DI H 2 O were mixed under stirring for 10 min. Thirdly, 1 ml K 3 PO 4 (0.1 M) and 9 ml H 2 O were stirred for 1 h at 60°C water bath. Then DNA, MnSO 4 , and K 3 PO 4 mixture was stirred for another 1 h until the solution became transparent which indicates Mn 2+ ions and phosphate groups of DNA reacted completely. The obtained solution was centrifuged at 9000 r/min for 10 min, and the DNA@ Mn 3 (PO 4 ) 2 pellet was collected. The supernatant was tested by UV-vis measurement at the range of 200-500 nm wavelength. Finally, the synthesized DNA@ Mn 3 (PO 4 ) 2 nanocomposite was dispersed in DI H 2 O.

Fabrication of DNA@Mn 3 (PO 4 ) 2 /GO Electrode
Fluorine-doped tin oxide (FTO) coated glass (2.5 cm × 1 cm, sheet resistance <10 Ω/sq.) as a work electrode was ultrasonically pre-cleaned with acetone and ethanol for 30 min and dried with nitrogen stream. Subsequently, oxygen plasma (Mycro Technologies Co., Ltd.) was used to clear the FTO surface for 5 min. Then 6 mg/ml GO homogenous solution was spin coating onto pre-cleaned FTO surface and dried in room temperature. Finally, DNA@Mn 3 (PO 4 ) 2 solution was further drop-casted on GO/FTO surface.

Characterization of Materials and Electrodes
Structural properties of synthesized nanocomposite were measured using JSM-6510LV scanning electron microscopy (SEM). JEM-2100 transmission electron microscopy (TEM) was further used to characterize the morphology in detail. GO and DNA were characterized by atomic force microscopy (AFM). P, N, O, and Mn elements were identified by X-ray photoelectron spectroscopy (XPS). FTIR transmittance spectra were obtained from NICOLET 6700. UV-2550 spectrophotometer recorded UV-vis absorption spectra, and the background contribution of the FTO glass to the absorption spectra was eliminated. The electrode of DNA@Mn 3 (PO 4 ) 2 /GO was examined by energy dispersive X-ray spectroscopy (EDS, equipment with JSM-6510 LV, Japan).

Photoelectrochemical Measurements
Photoelectrochemical measurements were carried out on an electrochemical workstation (CHI 760E, Chen Hua Instruments Co. Ltd.). A three-electrode system was employed with a FTO as a working electrode (photoelectrode), an Hg/HgCl 2 /KCL electrode (saturated calomel reference electrode, SCE) as a reference electrode, and a 2.5 cm 2 platinum plate as a counter electrode. All tests were conducted with a self-built 2 cm × 2 cm × 1 cm quartz electrolytic cell in 0.01 M phosphate-buffered saline (PBS). Ultra violet light Source (MUA-165, Japan) was used to irradiate the work electrode under UV light (λ: 180-420 nm, 15 mW/cm 2 ). The experimental measurements were carried out at room temperature without seal. Linear sweep voltammetry (LSV) was recorded at a scan rate of 50 mV/s. The amperometric i-t curve was recorded to measure photocurrent response under open circuit potential-time with intermittent light irradiation.

Unique Morphology and Properties of DNA@Mn 3 (PO 4 ) 2 /GO
The synthesis processes of DNA@ Mn 3 (PO 4 ) 2 nanocomposite and DNA@ Mn 3 (PO 4 ) 2 /GO electrode are schematically shown in Scheme 1. The DNA was used to guide the crystals nucleation of Mn 3 (PO 4 ) 2 nanoparticles. In the presence of Mn 2+ solution, the rich negatively charged PO 4 3− groups regularly arranged on the sugar-phosphate backbone of DNA attracted Mn 2+ ions onto the surface of DNA molecule strains. The following growth of Mn 3 (PO 4 ) 2 was promoted by potassium phosphate (K 3 PO 4 ). Due to the low solubility of Mn 3 (PO 4 ) 2 , the nanoparticles precipitated in the solution. Assembling of DNA@ Mn 3 (PO 4 ) 2 nanoparticles onto GO nanosheets was achieved by π-π non-covalent conjugation and electrostatic interaction between the base pairs of DNA and the functionalized surface of GO nanosheets.
TEM images of GO nanosheets are shown in Fig. 1a, b. The selected area electron diffraction (SAED) pattern of GO is shown as the inset of Fig. 1a. The well-defined diffraction cycles confirm the integrity of GO sheets. The GO thickness is in nanometer scale (Additional file 1: Figure S1A), as thin as the yarn, and there are some wrinkles covering the electrode surface. After the drop-coating of DNA@Mn 3 (PO 4 ) 2 , the DNA@Mn 3 (PO 4 ) 2 nanoparticles are dispersed onto the surface of GO sheets uniformly (Fig. 1c). The DNA@ Mn 3 (PO 4 ) 2 nanoparticle size is about 20-50 nm. HRTEM image shows regular lattice fringes, indicating Mn 3 (PO 4 ) 2 nanoparticle has relative good crystal quality (Fig. 1d). The fringe spacing as shown in Fig. 1d (Fig. 2c) appearing at binding energy of 532.1 eV is originally from the phosphate groups in DNA@Mn 3 (PO 4 ) 2 /GO. The N 1s peak (Fig. 2d) can be identified at 399.9 eV, directly demonstrating the existence of DNA molecules in hybrid electrodes. The peak of 133.5 eV can be attributed to P 2p (Fig. 2e) of phosphate. The XPS spectra of N 1s and P 2p peaks can fully demonstrate DNA adsorbed on the surface of the electrode [28]. This indicates that Mn 2+ ion has combined with phosphate radical of DNA and it plays a key role for material synthesis.
FTIR was used to further characterize the DNA@Mn 3 (PO 4 ) 2 /GO (Fig. 3a). The prominent characteristic peaks at about 3200 cm The optical absorption properties of different electrodes were revealed by UV-vis absorption spectra in 200-600 nm (Fig. 3b). Two features of GO absorption plot (green line) can be used to identify GO: a maximum at 231 nm, which denotes π-π* transition peaks of aromatic C-C bonds, and a gentle bump at about 300 nm [30] was corresponding to n-π* transition shoulder peaks of C = O bond [29]. The two peaks have certain red shift by conjugation. The ssDNA (blue line) electrode has a strong feature absorption peak at 260 nm [31]. Whereas Mn 3 (PO 4 ) 2 has a weak absorption peak near 260 nm, indicating a wide band gap of Mn 3 (PO 4 ) 2 . UV-vis absorption spectra of ssDNA@Mn 3 (PO 4 ) 2 shows lower absorbance peaks than DNA, suggesting the phosphoric acid groups on ssDNA reacted with Mn 2+ ions. While a peak at 231 nm of DNA@Mn 3 (PO 4 ) 2 /GO is further observed, attributing to the π-π* accumulation of GO and DNA. Furthermore, the amount of DNA in the DNA@Mn 3 (PO 4 ) 2 composite was quantified by comparing the absorbance value at 260 nm of the DNA solution before and after forming Mn 3 (PO 4 ) 2 precipitates. As shown in Additional file 1: Figure S3A, after reaction with MnSO4 and K 3 PO 4 , the absorbance value of the solution decreased. Based on the DNA calibration curve (Additional file 1: Figure S3B), the decreased DNA quality is 6.6 μg. The photocurrent density of GO/ssDNA@ Mn 3 (PO 4 ) 2 is obviously higher than other electrodes. However, the dark current density is also higher, indicating the corrosion maybe happen on this electrode. This should be detail investigated in further research. As shown in Fig. 4b, the GO/ssDNA@Mn 3 (PO 4 ) 2 electrode can deliver applied bias photon-to-current efficiency of~0.18%, while GO, ssDNA, and GO/ssDNA electrodes only achieved 0.03, 0.01, and 0.019%, respectively. Photocurrent responses from all electrodes indicate that a transient photocurrent increase after  Figure S2) and TEM. a, b Low-magnification and high-magnification TEM images of GO sheets. Inset of Fig. 1a is the SAED pattern of GO sheets. c Low-magnification TEM image of DNA@Mn 3 (PO 4 ) 2 /GO. d HRTEM image of Mn 3 (PO 4 ) 2 nanoparticle. Inset of Fig. 1d is the SAED pattern of DNA@Mn 3 (PO 4 ) 2 /GO UV irradiation. Furthermore, no visible charge recombination was observed from the photoelectrochemical measurements.

Photoelectrochemical Behaviors of DNA@ Mn 3 (PO 4 ) 2 /GO
The photoelectrochemical behaviors were investigated in 0.01 M PBS (pH = 7.4) with three-electrode configuration (Additional file 1: Figure S4). The LSV and amperometric i-t techniques were used to analysis electrode photoelectrochemical behaviors. Additional file 1: Figure S4a and b shows the light responses with various DNA molecules. The photocurrent densities of dsDNA and lmDNA are 6  and 5 μA/cm 2 , respectively, whereas the photocurrent density of ssDNA can reach up to 9 μA/cm 2 . Therefore, the ssDNA can deliver higher photoelectric response under UV light irradiation. Furthermore, we optimized the GO concentration and the results are shown in Additional file 1: Figure S4c and 4d. 6 mg/ml GO solution can obtain current density of 9 μA/cm 2 , whereas 3 and 10 mg/ml can only reach 5 and 3 μA/cm 2 , respectively. GO is a p-type semiconductor, therefore only a suitable GO concentration (6 mg/ml) could obtain good electrical behaviors and good photocurrent response. Mn 2+ ions concentration is another important factor, which can influence the performance of electrodes. As shown in Additional file 1: Figure S4e Figure S5). Under UV light irradiation, the electron-hole pairs can be generated in GO, DNA, and Mn 3 (PO 4 ) 2 , resulting from the high energy of UV light (2.95-6.89 eV). The excited electrons in Mn 3 (PO 4 ) 2 can be further excited and transferred to DNA. And then, the excited electrons can transfer as the gradient of energy level. At last, the electrons collected through FTO electrode were used to catalyze the water splitting on Pt counter electrode. Meanwhile, the photogenerated holes are easy to transfer/transport through GO and DNA due to their p-type properties. Then

Discussion
To investigate the enhanced photoelectric conversion efficiency of nanoparticle photoelectrodes in this work, we firstly fabricated the DNA-based nanoparticle photoelectrode (DNA@Mn 3 (PO 4 ) 2 /GO) for photoelectrochemical solar cells (PECs). DNA was often used as a template to guide the nucleation and growth of nanoparticle materials [14]. DNA and GO can bind together by strong interactions including van der Waals, π-π stacking and combination of hydrogen. Therefore, we used the DNA as a soft template to guide the synthesis of Mn 3 (PO 4 ) 2 nanoparticles with the size in nanoscales. Recent reports revealed that the Mn-based oxide catalyst could deliver a significant OER electrocatalytic performance [32]. GO sheets with the p-type semiconductor characteristic can efficiently reduce the recombination, while Mn 3 (PO 4 ) 2 can harvest the UV light largely. DNA@Mn 3 (PO 4 ) 2 /GO electrodes provide well-matched energy levels to improve the photoelectron transfer from the semiconducting film to the electrode and reduce the recombination of photoelectrons and holes.
In order to further enhance the photoelectric conversion efficiency, we optimized the molecular weight of the DNA, the GO concentration, Mn ion concentration. Under UV light irradiation (180-420 nm, 15 mW/cm 2 ), the photocurrent density of DNA@Mn 3 (PO 4 ) 2 Mn 3 (PO 4 ) 2 /GO electrodes reached 9 μA/cm 2 at 0.7 V bias (vs. SCE). An ABPE efficiency of~0.18% can be achieved-much higher than that of other control electrodes (<0.04%). Thus, we believe our results represent a potential application in enhancing the photoelectric conversion efficiency of nanomaterials.

Conclusions
In summary, we have developed a novel DNA@Mn 3 (PO 4 ) 2 /GO photoelectrode supported with GO sheets. The DNA@Mn 3 (PO 4 ) 2 /GO electrodes possess sensitive and higher response to ultraviolet light. Under UV light irradiation (λ: 180-420 nm, 15 mW/cm 2 ), the photocurrent intensity of DNA@Mn 3 (PO 4 ) 2 /GO electrodes can reach up to 9 μA/cm 2 at 0.7 V bias (vs. SCE), corresponding to applied bias photon-to-current efficiency of 0.18%, which is higher than other different electrodes prepared for comparison. DNA@Mn 3 (PO 4 ) 2 /GO electrodes provide a well-matched energy level to improve the photoelectron transfer rate from the semiconducting film to the electrode and reduce the recombination of photoelectrons and holes. This work demonstrates that DNA behaved as a useful biomaterial for the synthesis of a photoelectroactive hybrid film with improved performance.