Once a semiconductor is irradiated with enough light energy to excite electrons at the conduction band (CB) to the valence band (VB), semiconductor-electrolyte electron transfer (et) may occur at a rate that depends on the difference between electrolyte RedOx potential (ERedOx) and the Fermi level of the semiconductor (EF), but also on the RedOx density of state (DOS)-semiconductor interaction. Electron-hole recombination (e/h) competes with the et, therefore its minimization is a desired target in semiconductor photocatalytic design. When EF < ERedOx, et is thermodynamically favorable; however, equilibrium condition (EF = ERedOx) must be established and electron depletion occurs at the semiconductor side due to the lower electron density at the semiconductor as compared to solution[1]. Therefore, EF must get more positive to reach ERedOx value. Since ECB (conduction band potential) and EVB (valence band potential) do not change, bending of the bands occurs producing charge gradient (space charge region) at the semiconductor interphase promoting charge transport and separation that affect the dynamics of photogenerated carriers. Steady-state and time-resolved photoluminescense spectroscopy provide information on the rate of photophysical deactivation of carriers. In general[2],[3], the lower photoluminescense the lower the recombination rate and higher for instance the et rate. Colloidal particles that form a transparent/translucent solution allow photoluminescence measurements once the colloidal solution is excited with UV-Visible light[4]. To observe luminescent sensing of electron acceptors (ERedOx), the lifetimes of radiative recombination and electron donation from the semiconductor and the receptor at the electrolyte must match, otherwise, the effect would not be observed. Therefore, it is not expected to observe this sensing effect in all semiconductors and solvent conditions. Bi2WO6 becomes a special case due to the positive conduction band potential that reduces the rates of electron donation and makes it comparable to the radiative e/h. We have reported[5],[6],[1] lifetimes (t) of photoluminescence decays under conditions of N2 bubbling to the solution and without bubbling N2 at pH = 7. Three t values can be obtained by adjusting the decays: t1 from the ECB to EVB decay and t2 and t3 from recombination from shallow and deep traps to EVB, respectively. The t1 values are 6.5 ns without oxygen and 3.1 ns with O2 ([O2] = 1mM). The last values involve contributions from et to O2 and e/h (electron hole) recombination. Since these contributions operate in parallel, the observed rate kobs = ket + ke/h = 1/tobs = 1/tet + 1/te/h; therefore, 1/3.1 = 1/tet + 1/6.5 from where tet = 6.2 ns is easy to calculate. This value is slightly smaller than the te/h = 6.5 ns. These lifetime values are in agreement with the condition of similar rates required to observe the sensing of the ERedOx value. In addition, we have synthesized[6] Bi2WO6-4SCA, where 4-stilbene carboxaldehyde (4SCA) has been added to Bi2WO6 to promote electron transfer to the delocalized organic moiety. Bi2WO6-4SCA t1 value under nitrogen bubbling condition is shorter (4.0 ns) than in the case of Bi2WO6. Therefore, electron transfer to 4SCA unit decreases the decay lifetime. This represents evidence for the et contribution to the observe t1 value.
In this report, an unprecedented ERedOx luminescent sensing possibility that allows the identification and quantification of electron acceptors using Bi2WO6 colloids suspended in a water solution is discussed. Bi2WO6 is of special interest since the relatively positive conduction band suggests that photophysical deactivation processes will occur unless some indicated acceptor appears. By a simple modulation of the reaction (e.g. variation of the nature of the electron acceptor and/or its concentration), the changes in the spectroscopic signal are expected, which is interesting for sensing applications. For example, this capability is particularly relevant in the detection of oxidants such as O2 and H2O2, which act as electron receptors in the glycolysis[7] process activated by cancer cells.
Before moving forward, it is important to indicate the potential values that characterize the key processes to be discussed. Band gap energy Eg = 2.5 eV has been obtained for Bi2WO6 using UV-Visible reflectance spectroscopy[5]. From electrochemical measurements, the ECB = +0.50 V vs. NHE has been obtained[8]. At pH = 7 the ERedOx of O2/H2O is 0.81 V vs. NHE[9]. The H2O2 one electron reduction H2O2/HO•,H2O (0.38 V vs. NHE) and two electrons reduction H2O2/2H2O (1.349 V vs. NHE) have been previously probed[10]. Using the Nernst equation at pH = 4: E = (1.23 - 0.0591 x pH) V vs. NHE, the potential of O2,H+/H2O: ERedOx = 1.0 V vs. NHE has been obtained, and for H2O2, H+/HO•,H2O, at pH = 3.7 (E = 0.80 - 0.0591 x pH) E = 0.58 V vs. NHE was calculated. In addition, it should be noted that the Bi2WO6 used in this publication is the same as that obtained in the synthesis already published by our group[5,6]. Briefly, the synthesis method involves the hydrothermal synthesis of Bi2WO6, with details on SEM image, EDX analysis, XRD diffractogram, FTIR spectrum, B.E.T. isotherm, UV-Vis reflectance spectrum, and band gap calculation using Kubelka-Munk theory which can be seen in the Supporting Information. The photoluminescence measurements conducted for the present work involved quantifying emission spectra at a constant excitation wavelength (270 nm) (Fluoromax4 Horiba). These measurements were carried out in a spectroscopic grade quartz cell with a wavelength scan of 1 nm. To ensure the reproducibility of the experimental results, all measurements were performed using the same suspension (3 ml of water and 700 ppm of Bi2WO6). To record the emission spectra, the excitation wavelength was fixed, then the emission spectrum was recorded by scanning the wavelength from 310 to 700 nm with an integration time of 0.1 s to acquire 196 data points. The front entry and exit slits were set to 4.00 nm. Regarding the photoluminescence results, the methodology included conducting experiments in the same quartz cell. Initially, the suspension was purged with N2 to remove O2, followed by acquiring a photoluminescence spectrum. Subsequently, the suspension was oxygenated with air (until dissolving 1 mM of O2 verified with Hanna O2 sensor), and another photoluminescence spectrum was acquired. Finally, H2O2 (1 mM) was added (after purging with N2), and another spectrum was acquired. This methodology guarantees the comparability of measurements as the same cell, volume, and quantity of Bi2WO6 were used in each experiment, and the suspension inside the quartz cell was consistently agitated during each measurement. Each spectrum arises as an average of 5 spectra. Spectral maxima were estimated using the FluorEssence™ program for Windows® which is based on the Origin® software. The time-resolved photoluminescence signal (FL3 TCSPC-SP Horiba) was measured at 440 nm at 0.05 ns intervals. The fitting protocol was performed using Horiba DAS6 decay analysis software. In addition, dynamic light scattering (DLS) analysis of the Bi2WO6 suspension in water was performed to obtain valuable information on the size distribution of the dispersed phase of the suspension. The photoluminescence results recorded in this study are associated with Bi2WO6 suspensions exhibiting the characteristics observed in the DLS results.
Luminescent sensing of electron acceptors in B2WO6 water colloids has been observed as follows. Emission at 441 nm = 2.81 eV (Figure 1, black and red plots) exactly corresponds to Eg + (ERedOx - ECB) = 2.5 + (0.81 - 0.5) = 2.81 eV, that is an e/h transition from the non-bended bands: ECB to a hole at EVB + (ERedOx - ECB) (Figure 2). Note that ECB = 0.5 V vs. NHE[8] and ERedOx = E(O2/H2O) = 0.81 V vs. NHE at pH 7[9]. At [O2] = 1 mM (16 mg/L), this signal intensity decreases by ca. 1.3 fold as compared to [O2] << 1 mM (Figure 1, red plot). A blue plot (pH = 4) is also shown in Figure 1. A decrease in intensity of ca. 2.6 fold is observed but also a shift in the main band energy from 441 nm to 413 nm = 3 eV. This energy corresponds to Eg + (ERedOx - ECB) = 2.5 + (1 - 0.5) = 3 eV (Figure 2) that exactly match with O2 as receptor since at pH = 4, ERedOx = 1 V vs. NHE for the O2,H+/H2O pair.
There is then a dependency of the Bi2WO6 main signal emission intensity on ERedOx; as [RedOx] increases, et also rises, which is associated with a favorable interaction between ECB and RedOx-DOS. This, in turn, leads to a reduction in the partition of e/h recombination rates and consequently results in a decrease in spectral intensity.
Now, to further support the fact that the observed energetic effects such as spectral shifts and intensity changes are related to the nature of the electron acceptor affecting the kinetics of both the photophysics and the photoredox processes, in Figure 3 the resulting photoluminescence transient, of the Bi2WO6-Water suspension with H2O2 ([H2O2] = 1 mM) is shown. Briefly, in the area of photoluminescence analysis related to decay intensity over time, multiple deactivation pathways have been documented[11]. One objective is to discern the role of at least three mechanisms of photophysical deactivation due to the decay of excited electrons from the conduction band to the valence band (t1), surface states (t2), and deep states (t3)[1] . The decay to three exponentials for Bi2WO6 has been reported in the literature[5,6]. Lifetimes' magnitudes depend on experimental conditions, and in this study, the results of decay lifetimes in H2O2 are incorporated. From its transient analysis the following lifetimes were obtained: t1 = 3.0 ns, t2 = 5.2 ns and t3 = 66.9 ns. The lifetime values in the presence of H2O2 are slightly lower than those reported when there is the presence of 1 mM O2 (t1 = 3.1 ns, t2 = 6.5 ns and t3 = 168 ns)[5] , and significantly lower when the dissolved O2 was removed (N2-bubbling) (t1 = 6.5 ns, t2 = 14.7 ns and t3 = 3.0 ms)[5]. This means that H2O2 accepts the CB-excited electrons better than O2. Quite better et is also detected from the surface and deep traps. These results determine that [RedOx] and its DOS may influence the signal intensity of both transient and steady-state photoluminescence.
As shown in Figure 1, green plot, when [H2O2] = 1 mM (pH = 3.7) is added to colloidal Bi2WO6-Water suspension, signal intensity decreases as expected due to the [H2O2]; however, instead of a blue shift (E (H2O2/H2O) = 1.349 V - 0.591 x 3.7 = 1.13 V vs. NHE) as compared to O2 at pH = 4, a shift to 480 nm = 2.58 eV is observed. This means that the one-electron reduction pair prevails: E (H+,H2O2/HO•,H2O) = 0.80 V - 0.0591 x 3.7 = 0.58 V vs. NHE, pH = 3.7. This value agrees with the energy Eg + (ERedOx - ECB) = 2.5 + (0.58 - 0.5) = 2.58 eV = 480 nm (Figure 2). Therefore, in the absence of HO• quencher, a driving force favors the one-electron transfer. HO• is formed by reduction of H2O2 but it is consumed quite fast in solution via recombination with the HO• produced at the VB oxidation to regenerate H2O2. In Figure 1 spectra, the small signal observed at 550 nm (2.25 eV) may be influenced by the second order signal due the harmonic of the light excitation at 270 nm, which is expected to contribute at 540 nm. According to the literature[12], around 550 nm, information on oxygen vacancy trap electron recombination (EVo) with a hole at the VB could be obtained, but this signal is expected to be of low intensity and overlapped with the laser harmonic, requiring further studies. The signal for oxygen vacancies has been reported around 550 nm for light activation of Bi2WO6 during CO2 reduction[12], which corresponds to EVo = 0.75 V vs. NHE (estimated value of the following equation: Eg - (EVo- ECB) = 2.25 eV).
It should be noted that the spectral acquisition time, where the photoluminescence phenomena are observed, is a few seconds (<60 s). Therefore, in the Supporting Information, emission spectra are provided, showing that the emission intensity decreases in the presence of O2 compared to when N2 is bubbled. These results were verified at different wavelengths consecutively, demonstrating that possible changes on the Bi2WO6 surface do not affect the spectroscopic signal and trends within the measurement time interval. In addition, no color change of Bi2WO6 has been observed after spectroscopical experiments.
As shown in Figure 2, the maximum absorption blue shift concerning the Bi2WO6 band gap and the corresponding sensing of the ERedOx position is explained in terms of ERedOx-assisted radiative Auger recombination. An electron at the CB is transferred to the electrolyte ERedOx value. The energy gained in the transfer is transmitted to an electron at the non-bended CB. This electron with the extra energy radiatively recombines with a hole at the non-bended VB. A similar mechanism has been proposed[13] with evidence for trap-assisted Auger recombination (TAAR) in ZnO nanocrystals. Since the two electrons (Figure 2) are not in contact, entangled of their energy states is proposed as the energy transfer mechanism (Figure 2, bottom). In the specific case of the luminescence observed at pH = 7, the entangled energy state could be described in V vs. NHE as: 1/20.5(|0.81>|2.5>+|2>|0.31>). With a similar description, it has been proposed[14],[15] that quantum entanglement can be used in a recyclable way for energy transmission. The energy state entanglement between electrons may be established by LSPR[12] (Localized Surface Plasmon Resonance) produced by the photoluminescence irradiation. LSPR in Bi2WO6 has been reported[16] in the selective reduction CO2 to methane and quantum entanglement mediated by epsilon-near-zero materials (ca. oxides nanomaterials) has been reported[17]. In addition, in the context of the discussion on multiple electron transfer (MET) directed by the light-intensity dependence (LID) of the kinetic law in the photocatalytic process[18], it is suggested that the valence band (VB) might provide enough energy to enable MET during the oxidation of water. There is also the proposal that one, two, or even four positive holes must accumulate within each photocatalyst particle for simultaneous transfer to water. Thus, LSPR may excite multiple electrons simultaneously to the CB originating their energy entanglement.
Using Bi2WO6-4SCA (pH = 7, [O2] << 1 mM), the photoluminescense maximum[6] is maintained at 441 nm (2.81 eV) nevertheless, as expected the intensity decreases due to the 4SCA moiety extension and electron delocalization (Figure 4). This means that the entanglement coherence among the electrons remains when increasing their distance and the signal intensity senses the transferred electron transition through the organic 4SCA.
At this point, it is worth mentioning that the discussed photoluminescence results correspond to the prepared Bi2WO6-Water suspension; therefore, associating DLS results is important. In fact, DLS is shown in Figure 5. Specifically, an asymmetric distribution is observed with a maximum hydrodynamic diameter around 18 nm, confirming that the suspension under study is a system where the hydrodynamic diameter of the dispersed phase is always less than 100 nm.
After this discussion, the following question arose: to what extent could the identified spectroscopic principle be applied for the detection of electron acceptors in relevant environments? Several authors have used the changes and behavior of the luminescence bands for the design of sensors with high sensitivity, such as for the determination of Grapevine virus A-type onto modified ZnO surfaces[19], porphyrin-modified TiO2 nanostructures for amino acid detection (egvaline)[20], TiO2-x/ TiO2-based hetero-structure to detect ethanol, methanol, n-propanol and acetone gases/vapours[21], antigens on the surface of TiO2 to sense retroviral leucosis[22] and protein modified TiO2 to detect Bovine Leucosis antibodies[23]. These studies show changes in the emission signal without analyzing in detail the signal shifts involved. Also associate the optical signal with the material defects or modifications, linking with the synthesis conditions[24]- [26], as well as the always-mentioned surrounding effect (e.g. solvochromic effect) to qualitatively justify any shift. However, the systematic observation of RedOx signaling of receptors like O2 and H2O2 described in this manuscript suggests a crucial role in photo-induced electron transfer-based luminescent sensing (e.g. in carcinogenesis[27]-[29]). For instance, trans-plasma membrane electron transfer (tPMET)[7] is activated by cancer cells to provide them with extra energy (ATP) via glycolysis. The reported here phenomena should be explored as a functional principle to design smart devices for sensing and treatment as shown in Figure 6. Aortic dissection (AD), a cardiovascular disease, is initiated by over expression of Reactive Oxygen Species (ROS) in the aorta that damage the vascular structure[30] causing death. Detecting ROS and implementing its minimization is another possible application for the findings described herein.
In summary, Bi2WO6 colloidal solution pholuminescence emits light at Eg + (ERedOx - ECB). This energy depends on electrolyte electron acceptor potential (ERedOx) and its signal intensity inversely depends on [RedOx] and electron transfer rate. Bi2WO6 similar rates of e/h recombination and electron transfer are crucial for the sensing to be detected. Energy gained from the exergonic electron transferred from the semiconductor conduction band to the acceptor in the electrolyte is transmitted to a second electron that decays emitting light producing the sensing effect. Since the two electrons are not in contact during the events, energy levels entanglement is proposed. Localized surface plasmon resonance may induce the corresponding energy entanglement. Possible application of this discovery on cancer cells detection and treatment is highlighted. The proposed mechanism is supported by observations: proportional shifts in photoluminescence spectrum with RedOx potentials, inverse signal intensities with RedOx concentrations, and unaffected sensing signals by electron distance. The simpler photosensitize/quenching mechanism lacks a clear rationale for energy transfer without close contact, but the model of continuous surface states offers insight. Analogy to a selective rectifying junction at the semiconductor-electrolyte interface further validates the mechanism, with observed electron transfer rates aligning with its unique characteristics in Bi2WO6.
