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A visible light-induced self-powered sensor for the detection of tyrosinase activity was proposed. A tyrosinase-responsive photoelectrochemical-chemical redox cycling strategy was integrated with a photofuel cell for signal amplification.

A series of novel Eu2+/3+, Ce3+ ion single- and co-doped BaZn2(PO4)2 samples were prepared via a high-temperature solid-state reaction. XRD powder diffraction results indicated that all of the products were pure phases. The photoluminescence properties of BaZn2(PO4)2:Eu showed that Eu2+ and Eu3+ coexist in the system and Eu3+ can be self-reduced to Eu2+ in an air atmosphere. In addition, the strongest emission peak of Eu3+ ions at 593 nm implied that Eu3+ ions occupy the inversion symmetry lattice and also the site of Zn in BaZn2(PO4). We used the theoretical method of bond energy to explain why the self-reduction of Eu3+ to Eu2+ can occur in the BaZn2(PO4)2 system. The calculation results indicated that the bond energy change value is smaller than , indicating that Eu2+ ions are more likely to occupy the Zn site and more stable than Eu3+ ions in BaZn2(PO4). Furthermore, the energy transfer process between Ce3+ and Eu2+ ions in the photoluminescence spectrum and the decay lifetime were observed, and the energy transfer mechanism was determined to be a dipole–dipole interaction. In this work, by adjusting the ratio of Ce and Eu ions, the emission color can be changed from blue to white, implying that the phosphor can be used as a promising candidate in the manufacture of white LEDs.

The vanadate-based phosphors Sr2V2O7:Eu3+ (SV:Eu3+), Sr9Gd(VO4)7:Eu3+ (SGV:Eu3+) and Sr9Gd(VO4)7/Sr2V2O7:Eu3+ (SGV/SV:Eu3+) were obtained by solid-state reaction. The bond-energy method was used to investigate the site occupancy preference of Eu3+ based on the bond valence model. By comparing the change of bond energy when the Eu3+ ions are incorporated into the different Sr, V or Gd sites, we observed that Eu3+ doped in SV, SGV or SV/SGV would preferentially occupy the smaller energy variation sites, i.e., Sr4, Gd and Gd sites, respectively. The crystal structures of SGV and SV, the photoluminescence properties of SGV:Eu3+, SV, SGV/SV and SGV/SV:Eu, as well as their possible energy transfer mechanisms are proposed. Interesting tunable colours (including warm-white emission) of SGV/SV:Eu3+ can be obtained through changing the concentration of Eu3+ or changing the relative quantities of SGV to SV by increasing the calcination temperature. Its excitation bands consist of two types of O2− → V5+ charge transfer (CT) bands with the peaks at about 325 and 350 nm respectively, as well as f–f transitions of Eu3+. The obtained warm-white emission consists of a broad photoluminescence band centred at about 530 nm, which originates from the O2− → V5+ CT of SV, and a sharp characteristic spectrum (5D0–7F2) at about 615 and 621 nm.

Ca10M(PO4)7:Eu (M = Li, Na, K) phosphors have been synthesized via a solid-state reaction process, their phase purity was examined using XRD patterns, and Rietveld refinement confirmed that the Ca10Li(PO4)7, Ca10Na(PO4)7 and Ca10K(PO4)7 are pure phases. The photoluminescence properties of the Ca10M(PO4)7:Eu (M = Li, Na, K) phosphors showed that the self-reduction of Eu3+ to Eu2+ can occur in an air atmosphere. Eu3+ ions can be reduced to Eu2+ ions when doped in Ca10Li(PO4)7, Ca10Na(PO4)7 and Ca10K(PO4)7 crystals, which was detected using photoluminescence spectra. In this work, the bond energy method was used to determine and explain the mechanism of site occupation of Eu entering the host matrix. According to the calculated value of the deviation of bond energy for Eu3+-doped Ca10M(PO4)7 (M = Li, Na, K) crystals, the similar value between and , and , and and can provide the conditions for the self-reduction of Eu3+ in the Ca10M(PO4)7 (M = Li, Na, K) system. Meanwhile, the smaller deviation values of , , and in Ca10Li(PO4)7, Ca10Na(PO4)7, and Ca10K(PO4)7 crystals and in Ca10K(PO4)7 crystals indicated that the preferential sites of Eu ion occupancy in the Ca10M(PO4)7 (M = Li, Na, K) lattices are Li, Na, K and Ca sites. The conclusions obtained from the calculated results of the bond energy method are consistent with the Rietveld refinement and the photoluminescence spectra of Ca10M(PO4)7 (M = Li, Na, K).

The local crystal sites occupied by Eu3+ in Ca5(BO3)3F:1% Eu3+ phosphor were investigated experimentally and theoretically. Ca5(BO3)3F:1% Eu3+ was synthesized by high-temperature solid-state method in air. The crystal structure and optical properties of the phosphor were studied by X-ray powder diffraction and photoluminescence, respectively. Two different O2− → Eu3+ CT broad bands with the peaks at 266 and 283 nm in Ca5(BO3)3F:1% Eu3+ were detected, indicating the Eu3+ sites occupied Ca2 and Ca1, respectively. The different sharp f–f emission spectra under the excitation of 283 and 266 nm proved that there are two different local lattice environments around Eu3+ existing in Ca5(BO3)3F:1% Eu3+. Environmental factor he, the standard deviation of environmental factor (EFSD) and the bond energy were used to illustrate and explain the site occupancy mechanism of Eu3+ into the host lattice. By comparing the intensity ratios of 5D0 → 7F2 transition to the 5D0 → 7F1 transition, I(5D0/7F2)/I(5D0/7F1) of Eu3+ at Ca2 (7.381) was found to be 2.5 times stronger than that of Eu3+ at Ca1 site (2.933). was calculated to analyze the I(5D0/7F2)/I(5D0/7F1) value. On the basis of the bond valence model, a bond-energy method was used to study the occupancy of the Eu ion, which indicated that the preferential sites of Eu ion occupancy in the Ca5(BO3)3F are the Ca2 and Ca1 sites. All three theoretical calculation results are consistent with each other.

CaY1−xAl1+xO4:2%Eu (x = 0, 0.1, 0.2) phosphors have been synthesized via a solid-state reaction process. XRD patterns indicate that they are pure phase. The photoluminescence properties of the CaY1−xAl1+xO4:2%Eu phosphors exhibit both the blue emission of Eu2+ (4f65d1–4f7) and red-orange emission of Eu3+ (5D0–7F1,2,3,4) under UV light excitation, which showed that the Eu3+/2+ co-doping system was obtained by adjusting the Al/Y ratio. Eu3+ ions can be reduced to Eu2+ ions when the Al/Y ratio was changed. In this work, the bond energy method was used to determine and explain the mechanism of the site occupation of Eu ions entering the host matrix. Also, the emission spectrum showed an unusual comparable intensity 5D0–7F4 transition peak. The relative intensity of 5D0–7F2 and 5D0–7F4 can be stabilized by changing the relative proportions of Al3+ and Y3+. Furthermore, this was explained by the J–O theory.