Exploring novel surface-enhanced Raman scattering (SERS) active materials with high detection sensitivity, excellent biocompatibility, low biotoxicity, and good spectral stability is urgently required for efficacious cancer cell diagnosis. Herein, black TiO2 nanoparticles (B-TiO2 NPs) with crystal–amorphous core–shell structure are successfully developed. Remarkable SERS activity is derived from the synergistic effect of the promising crystal–amorphous core–shell structure. Abundant excitons can be generated by high-efficiency exciton transitions in the crystal core, a feature that provides sufficient charge source. Significantly, the novel crystal–amorphous heterojunction enables the efficient exciton separation at the crystal–amorphous interface, which can effectively facilitate charge transfer from the crystal core to the amorphous shell and results in exciton enrichment at the amorphous shell. Kelvin probe force microscopy (KPFM) confirms the Fermi level of the amorphous layer shifting to a relatively low position compared to that of the crystal core, allowing efficient photoinduced charge transfer (PICT) between the amorphous shell and probe molecules. The first-principles density functional theory (DFT) calculations further indicate that the amorphous shell structure possesses a narrow band gap and a relatively high electronic density of state (DOS), which can effectively promote vibration coupling with target molecules. Moreover, MCF-7 drug-resistant (MCF-7/ADR) breast cancer cells can be quickly and accurately diagnosed based on the high-sensitivity B-TiO2-based SERS bioprobe. To the best of our knowledge, this is the first time the crystal–amorphous core–shell heterojunction enhancement of the TiO2-molecule PICT process, which widens the application of semiconductor-based SERS platforms in precision diagnosis and treatment of cancer, has been investigated.

SummaryExploring highly surface-enhanced Raman scattering (SERS)-active semiconductors is urgently required for practical applications. Here, with the guidance of theoretical calculations, amorphous rhodium sulfide microbowls with high enhancement factor (1 × 105) and low limit of detection (10−7 M) for rhodamine 6G are successfully developed. This remarkable sensitivity is attributed to quasi-resonance Raman effect and multiple light scattering. The first-principles calculations show that the energy gap of 4-nitrobenzenethiol adsorbed on Rh3S6 is greatly decreased by shifting its lowest unoccupied molecular orbital (LUMO) energy level close to the LUMO of Rh3S6, enabling quasi-resonance Raman effect by visible light. The finite-difference time-domain simulations demonstrate the efficient photon trapping ability enabled by multiple light scattering. The optimum wavelength of ∼633 nm for SERS is predicted in simulations and confirmed in experiments. Our results provide both a deep insight of the photo-driven charge transfer process and an important guidance for designing SERS-active semiconductors.

Enriching the electronic density of states (DOS) of semiconductors is the key to promoting charge transfer (CT) and achieving a large surface-enhanced Raman scattering (SERS) enhancement. Metal hydroxide semiconductors are anticipated to exhibit DOS that are higher than those of metal oxide because of their abundant O atoms; however, their SERS activity has not been verified. Here, combining density functional theory and experiments, we report a SERS sensitivity of amorphous Zn­(OH)2 [a-Zn­(OH)2] that is much higher than that of amorphous ZnO (a-ZnO), ascribed to the abundant O atoms and hence enriched O 2p state density near the Fermi level in a-Zn­(OH)2, which gives rise to higher CT probabilities. Moreover, we find a-Zn­(OH)2 exhibits significant advantages in energy-level matching over a-ZnO for efficient photoinduced CT via strong vibronic coupling, ascribed to the upshifted valence band maximum and the narrower band gap of a-Zn­(OH)2. Via the synthesis of a-Zn­(OH)2 nanocages, an ultrahigh enhancement factor of 1.29 × 106 is obtained in semiconductor-based SERS.

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