Topotactic Conversion of α-Fe2O3 Nanowires into FeP as a Superior

Jan 31, 2017 - Topotactic Conversion of α-Fe2O3 Nanowires into FeP as a Superior Fluorosensor for Nucleic Acid .... Materials Letters 2017 209, 48-51...
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Topotactic Conversion of #-Fe2O3 Nanowires into FeP as a Superior Fluorosensor for Nucleic Acid Detection: Insights from Experiment and Theory Li Yang, Danni Liu, Shuai Hao, Fengli Qu, Ruixiang Ge, Yongjun Ma, Gu Du, Abdullah M. Asiri, Liang Chen, and Xuping Sun Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04760 • Publication Date (Web): 31 Jan 2017 Downloaded from http://pubs.acs.org on February 1, 2017

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Analytical Chemistry

Topotactic Conversion of α-Fe2O3 Nanowires into FeP as a Superior Fluorosensor for Nucleic Acid Detection: Insights from Experiment and Theory Li Yang,† Danni Liu,† Shuai Hao,† Fengli Qu, Ruixiang Ge, †,§ Yongjun Ma,‡ Gu Du,∫ Abdullah M. Asiri,║Liang Chen,§,* and Xuping Sun†,* ┃

†

College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China. Tel/Fax: 0086-28-85412198. E-mail: [email protected] ┃

College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, Shandong, China Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, Zhejiang, China. E-mail: [email protected] ‡ Analytical and Test Center, Southwest University of Science and Technology, Mianyang 621010, Sichuan, China ∫ Chengdu institute of Geology and Mineral Resources, Chengdu 610081, Sichuan, China ║ Chemistry Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia §

ABSTRACT: Nanostructures possess distinct quenching ability toward fluorophores with different emission frequencies and have been intensively used as nanoquenchers for homogenous nucleic acid detection. Complete understanding of such sensing system will provide guiding significance for the design of superior sensing materials, which, however, is still lacking. In this Letter, we demonstrate the development of FeP nanowires as a nanoquencher for high-performance fluorescent nucleic acid detection with much superior performance to α-Fe2O3 counterparts. The whole detection process is complete within 1 min and this fluorosensor presents a detection limit as low as 4 pM with strong discrimination of single-point mutation. Electrochemical tests and density functional theory calculations reveal that FeP NWs are superior in both conductivity for facilitated electron diffusion and hydrogen-evolving catalytic activity for favorable electron depletion, providing further experimental and theoretical insights into the enhanced sensing performance of FeP nanosensor. Both faster electron transfer kinetics and stronger electron-consuming ability via catalyzed proton reduction enable FeP nanowires a superb nucleic acid nanosensor for applications.

Biosensors play important roles in early disease diagnostics, food safety, and environmental monitoring.1-5 Among them, nanosensor for sequence-specific detection of nucleic acid sequences associated with genetic and pathogenic diseases is of vital significance in molecular diagnostics.6,7 Compared to heterogeneous fluorescent assays, the homogeneous ones have disadvantages of convenient operation, rapid binding kinetics, and easy automation.8 Nanostructures have distinct quenching ability toward fluorophores with different emission frequencies9 and recent years have witnessed the rapid scientific advance and successful development of various such nanoquenchers for fluorimetric nucleic acid detection, including Au nanoparticle,10 nanocarbon,11-14 organic polymer nanostructure,15,16 transition metal sulfide nanosheet,17-20 and metal–organic framework nanosheet,21 etc. All these fluorosensors however suffer from high cost of noble-metal quencher, time-consuming detection process, limited structural stability, and/or unsatisfactory sensitivity and selectivity. Moreover, the quenching mechanism (energy or electron transfer) also remains controversial. Until recently have we established that photoinduced electron transfer (PET) from the excited dye labeling the oligonucleotide probe to the

semiconductor CoP nanowires as the quencher is responsible for fluorescence quenching.22 The design of superior sensing materials requires complete understanding of this sensing system, which, however, is still lacking. Fe is the cheapest and one of the most abundant transition metals on earth and in human body,23 promising its use for constructing cost-effective environmental friendly sensing materials. To gain better performance of PET-based fluorosensors, an ideal nanoquencher should satisfy the following two requirements: (1) it has high conductivity for rapid electron diffusion away from the contact region of quencher and dye; (2) such electron is immediately depleted to avoid accumulation on its surface for accelerated electron transfer. To the best of knowledge, however, Fe-based such nanosensor has not been reported before. In this Letter, we describe our recent effort toward this direction in developing FeP nanowires (FeP NWs), topotactically converted from α-Fe2O3 NWs, as a nanoquencher for high-performance nucleic acid detection within 1 min with superior sensing performance to α-Fe2O3 1

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1C), which is further supported by transmission electron microscopy (TEM) observations (Figure 1D and 1E). The high-resolution TEM (HRTEM) image taken from FeP (Figure 1F) shows well-resolved lattice fringes with interplanar distances of 2.73 Å and 2.14 Å indexed to the (011) and (111) planes of FeP, respectively. Figure 1G and 1H show the X-ray photoelectron spectroscopy (XPS) spectra for FeP in the Fe 2p3/2 and P 2p regions, respectively. The binding energy (BE) of 707.5 eV is characteristics of Fe in FeP and the peak at 710.9 eV can be assigned to oxidized Fe species arising from superficial oxidation of FeP due to air contact. The high-resolution P 2p region shows two peaks at 130.3 and 129.7 eV reflecting the BEs of P 2p1/2 and P 2p3/2, respectively, along with one peak at 133.6 eV. The peak at 129.7 eV is assigned to P in FeP and the peak at 133.6 eV can be attributed to oxidized P species. All these observations confirm the successful topotactic conversion of α-Fe2O3 NWs into FeP NWs.24,32

NWs. This fluorosensor shows a low detection limit of 4 pM with remarkably high discrimination ability toward single-point mutation. Electrochemical tests and density functional theory (DFT) calculations show that FeP has higher electron-transferring and hydrogen-evolving ability than α-Fe2O3, which provide further experimental and theoretical insights into its superior sensing performances.

Figure 1. A) XRD patterns of α-Fe2O3 (curve a) and FeP (curve b). SEM images of B) α-Fe2O3 and C) FeP. TEM images for one single nanowire of D) α-Fe2O3 and E) FeP. F) HRTEM image taken from FeP. XPS spectra of FeP in the G) Fe 2p and H) P 2p regions.

FeP NWs were derived from α-Fe2O3 NWs using established low-temperature phosphidation strategy.24 Such topotactic conversion reaction features the retetion of precursor morphology and this strategy has been widely used to make nanostructures of various transition metal phosphides (TMPs) under controlled conditions.24-30 Figure 1A shows the X-ray diffraction (XRD) patterns of the precursor (curve a) and the phosphidated product (curve b). The precursor shows diffraction peaks characteristics of α-Fe2O3 phase (JCPDS No. 33-0664).31 The phosphided product presents several diffraction peaks at 30.8°, 32.7°, 34.5°, 35.5°, 37.2°, 46.3°, 47.0°, 48.3°, 50.4°, 56.1°, 59.6° and 79.2°, which can be indexed to the (020), (011), (200), (120), (111), (121), (220), (211), (130), (031), (002) and (222) planes of FeP phase (JCPDS No. 39-0809), respectively. Scanning electron microscopy (SEM) analysis concludes that the precursor exclusively consists of α-Fe2O3 NWs with diameters ranging from 70 to 90 nm (Figure 1B) and the resulting FeP still preserves 1D morphology but with roughed surface (Figure

Figure 2. A) Fluorescence spectra of PHIV under differrent conditions: PHIV; PHIV+T1; PHIV+T1+FeP NWs; PHIV+FeP NWs; FeP NWs. B) Fluorescence spectra of PHIV+T1 with different T1 concentrations with the presence of FeP NWs (inset: amplification of the concentration range from 0 to 20 nM). C) Corresponding calibration curve (inset: amplification of the low-concentration range). D) Kinetic study of the fluorescence change of PHIV and PHIV/T1 in the presence of FeP NWs. E) Fluorescence spectra of PHIV/Tx with the presence of FeP NWs (x=1, 2, or 3). F) Fluorescence intensity changes [(F-F0)/F0] of PHIV and PHIV/Tx with the presence of FeP NWs (x=1, 2, or 3; F0 and F are the fluorescence intensity of PHIV and PHIV/Tx, respectively). Excitation was at 480 nm and the emission was monitored at 519 nm. All measurements were done in Tris-HCl buffer containing 100 mM NaCl, 5 mM KCl, and 5 mM MgCl2 (pH: 7.4). [PHIV]=10 nM, [FeP]=0.7 µg/mL, [T1]=[T2]=[T3]=100 nM. 2

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Analytical Chemistry great benefit to prompting electron transfer for improved sensing performance in a PET-based sensor. We compared the electrocatalytic HER activities of FeP NWs and α-Fe2O3 NWs deposited with the same loading of 0.7 mg cm-2 on glassy carbon electrodes in 1.0 M phosphate buffer solution (PBS) using a typical three-electrode setup. The ohmic potential drop losses from the solution resistance were applied to all initial data34 and all potentials were reported on a reversible hydrogen electrode (RHE) scale. Figure 3A shows the linear sweep voltammetry (LSV) curves of FeP NWs and α-Fe2O3 NWs. α-Fe2O3 NWs are active for the HER with the need of overpotential of 360 mV to drive 20 mA cm-2. In contrast, FeP NWs show greatly enhanced HER activity for the same current density with much lower overpotential of 240 mV. Figure 3B presents the Tafel plots of FeP NWs and α-Fe2O3 NWs. Tafel slope for FeP NWs (125 mV dec-1) is much lower than that of α-Fe2O3 NWs (181 mV dec-1), implying FeP NWs have a favorable catalytic kinetics for the HER. Electrochemical impedance spectroscopy results further show that FeP NWs possess a much smaller radius of semicircle than that for α-Fe2O3 NWs (Figure 3C), suggesting a higher charge-transfer rate and therefore markedly faster HER kinetics of FeP NWs.35

To evaluate the sensing performance of FeP NWs for fluorescent nucleic acid detection, we chose an oligonucleotide sequence associated with human immunodeficiency virus (HIV) as a model system. α-Fe2O3 NWs were also tested for comparison. Figure 2A presents the fluorescence emission spectra of PHIV under different conditions. PHIV, a fluorescein-based dye (FAM)-labled fluorescent probe, shows intense fluorescence emission arising from FAM dye. Incubation of PHIV with a large excess of complementary strand (T1) produces a PHIV/T1 duplex which almost has no impact on the fluorescence of PHIV. It is important to mention that FeP NWs exert distinctly different influence on the fluorescence intensity of PHIV and PHIV/T1. With the presence of FeP NWs, the fluorescence of PHIV is largely quenched (82%), implying FeP NWs adsorb PHIV with effective quenching of the fluorescent dye. PHIV has rich free nucleobases and the big difference in electronegativity between Fe and N leads to electron transfer from Fe to N. The resulting noncovalent interactions are believed to mainly contribute to PHIV adsorption on FeP.22 The fluorescence of PHIV/T1, however, is slightly quenched by FeP NWs (10%). Compared to FeP NWs, α-Fe2O3 NWs however have decreased quenching efficiency (75%), as shown in Figure S1A. Noted that both FeP NWs and α-Fe2O3 NWs are not fluorescent with no contribution to the whole fluorescence intensity. The fluorescence emission spectra of PHIV+T1 with different T1 concentrations in the presence of FeP NWs or α-Fe2O3 NWs were collected to evaluate the sensitivity. As observed, an increase in T1 concentration leads to intensified retention of the fluorescence intensity for FeP NWs (Figure 2B) and α-Fe2O3 NWs (Figure S1B). According to the calibration curves, FeP NWs show a linear range from 0.02 to 20 nM (Figure 2C) and α-Fe2O3 NWs have a linear relationship in the range of 1-50 nM (Figure S2A). And the nonlinear character of the calibration curve over the entire concentration range is probably due to that the increased T1 concentration causes more hybridization events near nanosensors surface.33 The detection limit of FeP NWs is 4 pM (three times the standard deviation in the blank solution), which is much lower than that of α-Fe2O3 NWs (1 nM) and all other reported non-noble-metal quenchers (Table S1).

Figure 3. A) LSV curves for FeP NWs and α-Fe2O3 NWs with a scan rate of 2 mV s-1 for HER in 1.0 M PBS. B) Tafel plots of FeP NWs and α-Fe2O3 NWs. C) Nyquist plots for FeP NWs and α-Fe2O3 NWs.

We collected time-dependent fluorescence emission spectra to probe the quenching kinetics of FeP NWs and α-Fe2O3 NWs toward PHIV and PHIV/T1. The whole detection process for FeP NWs sensor is complete within 1 min (Figure 2D), much shorter than that need by α-Fe2O3 NWs (over 15 min, Figure S2B) and other reported sensors (Table S1). Also note both sensors are capable of discriminating complementary and single-base mismatched targets (T2), as shown in Figure 2E and Figure S3A. For FeP NWs, the value for T2 is about 12% of the value for T1 while noncomplementary T3 demonstrates quite small change in fluorescence intensity (Figure 2F). Thus, FeP NWs provide excellent selectivity toward single-point mutation superior to α-Fe2O3 NWs (39%, Figure S3B) and reported CoP NWs (15%).22

To better understand the improved HER activity of FeP NWs, we applied DFT to calculate the electronic structures and hydrogen adsorption free energy (∆GH*) for both catalysts. ∆GH* is regarded as a key descriptor for the HER performance, and it is well accepted that an optimal HER catalyst should have a ∆GH* close to zero.36 By comparing the density of states (DOS) plots in Figure 4A and Figure 4B, it is clearly seen that α-Fe2O3 is a semiconductor with a band gap of approximately 2.02 eV, whereas FeP is metallic with no band gap. The continuous band states near the Fermi level guarantee fast electron transfer,37 serving as a grounded indicator for engineering the electronic structure of catalysts with enhanced HER activitiy.38 The outnumbered DOS around the Fermi level of FeP suggests enhanced electron mobility in comparison with α-Fe2O3. Moreover, the adsorption of H on α-Fe2O3 yields a ∆GH* of -2.13 eV (Figure 4C) while FeP has a

From a point view of catalysis, an electrocatalyst with higher activity toward hydrogen evolution reaction (HER) via proton electroreduction favors electron depletion, which is of 3

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much more thermo-neutral ∆GH* of -0.06 eV (Figure 4D). These theoretical results rationalize the superior HER activity of FeP over α-Fe2O3. Both the rapid electron transfer kinetics and high HER activity of FeP leads to effective depletion of the electrons transferred from photoexcited dyes, which accelerates the electron transfer rate at the interface. As a result, a shorter detection time and improved sensitivity were achieved. It is worthwhile mentioning that CoP, another well-studied TMP, has a ∆GH* of -0.14 eV which is also much more thermo-neutral than that for Co(OH)2 (1.15 eV). As a fluorosensor for nucleic acid detection, CoP NWs have a detection limit of 100 pM with a detection lime of 14 min.22 In contrast, Co(OH)2 NWs (Figure S4) show inferior sensing performance with a detection limit and a detection time of 0.7 nM and 30 min, respectively (Figure S5). The difference in ∆GH* for FeP and CoP also justifies the superior sensing performance of FeP to CoP. It is thus rational to conclude that this study would open up a new avenue in computer-aided rational design28,39 of hydrogen-evolving catalysts toward the development of PET-based sensing systems for applications.

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Experimental section, fluorescence spectra, calibration and kinetic curves, Table S1, histograms, and SEM image. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (L.C.); [email protected] (X.S.)

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant Nos. 21575137 and 21375076).

REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)

Figure 4. Total and projected DOS of A) α-Fe2O3(0001) and B) FeP(101) (black lines: the total DOS, blue lines: the d-orbital contribution of Fe, red lines: the p-orbital contribution of O in α-Fe2O3 and P in FeP). The DOS is shifted so that the Fermi level is set to zero. Schematic diagram to illustrate H adsorption on C) α-Fe2O3(0001) and D) FeP(101) surface (O, H, P, and Fe atoms are colored red, white, pink, and blue, respectively).

(14) (15) (16) (17)

In conclusion, FeP NWs have been proved as a superb fluorosensor over α-Fe2O3 for nucleic acid detection. The whole detection process is complete within 1 min, with a low detection limit of 4 pM and an extraordinarily high discrimination ability toward single-point mutation. Both electrochemical tests and DFT calculations demonstrate that FeP NWs have excellent conductivity for facilitated electron diffusion and high HER activity for favorable electron depletion, providing further experimental and theoretical understandings of the superior sensing performance. This study is important because it provides us a cost-effective high-performance fluorosensor for the detection of biomacromolecules and metal ions.12,14,40-42

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