A Flexible Broadband Visible Light Plasmon‐Absorber Based on a 2D Monolayer of Au‐Nanoparticle/TiO2‐Nanowire Core–Shell Heterostructures

Plasmon‐induced hot carrier transfer is a promising approach for photoelectric applications, but its practical application is often hindered by its relatively narrow absorption bandwidth and low light harvesting at visible wavelengths. This work reports a broadband visible light plasmon‐absorber based on a dandelion‐like hierarchical metal‐semiconductor Au‐TiO2 core–shell nanostructure where each Au nanoparticle (AuNP) core is covered by an optically transparent mesh‐shell composed of TiO2 nanowires (TNWs) (denoted as AuTNW‐dCS). These AuTNW‐dCSs are assembled to form a 2D arrayed plasmonic monolayer by connecting adjacent AuNPs with cross‐linked TiO2 NWs, which can induce a remarkable broadband absorption in the visible spectrum of 500–700 nm. Driven by a wide light adsorption range, improved hot‐electron carriers, and excellent analyst's immobilization capability, the AuTNW‐dCS based photoelectric immunosensor owns excellent performance for alpha‐fetoprotein detection in human serum, with a practical linear range of 0.1–1000 ng mL−1, a low detection limit of 0.1 ng mL−1, and satisfying selectivity under visible light‐emitting diode light irradiation. This work enlightens the prospective research on the use of metal/semiconductor core–shell heterostructures as photoactive materials for sensing applications. Additionally, it gives an impetus for developing flexible nanofilms based on plasmonic metal/semiconductor nanohybrids, beneficial for building wearable point‐of‐need platform.


Introduction
Hepatocellular carcinoma (HCC) is one of the most common cancer that starts in the human's liver, which represents a global health challenge. [1,2]Alpha-fetoprotein (AFP) is a wellknown biomarker for HCC diagnosis.It is demonstrated that an DOI: 10.1002/admi.202300415abnormal elevation of the AFP concentration would appear in early cancer stage in adult serum which would make a significant association with cancer lesions. [3,4]ence, early detection of the abnormal elevation of AFP concentration is critical for early diagnosis of HCC.At present, various techniques, including surfaceenhanced Raman scattering (SERS), [5][6][7] enzyme-linked immunosorbent assay (ELISA), [8,9] electrochemiluminescence immunoassay, [10,11] and electrochemical detection, [12,13] have been developed for the analysis of biological and chemical molecules.These methods are confirmed to realize sensitive detection and high accuracy.However, they still involve some disadvantages, such as relatively sophisticated instruments, high cost, too complicated for instant analysis, and so on.
Photoanode-supported photoelectric bioassay system has been widely employed for biosensing techniques recently due to its features of the coupled well-known merits in optical and electrochemical techniques and the particular property of effective differentiation between excitation light source and testing signal. [14,15]For practical applications, the photoanode materials should be stable, low-cost, and easy for large-scale production.Most importantly, it should possess a broadband absorption in the visible light range of 400-800 nm. [16][23][24][25] Also, the engineering Schottky junctions in plasmonic Au/TiO 2 heterojunctions can facilitate efficient plasmon-induced hot electron-hole generation and extraction at Au-TiO 2 interface, leading to improved photoactive performances. [26,27][30][31][32] 2D arrayed metallic monolayers show great potential in solar-to-energy conversion. [22,33]Therefore, it can be recognized that the optical absorption capacity of Au/TiO 2 can be greatly improved by the combination of TiO 2 with Au nanoparticle (AuNP) 2D arrayed plasmonic absorber with a wide broadband spectrum, especially in the visible region.As a notable example, Shi et al. reported that the AuNP/TiO 2 /Au-film layer-bylayer system was able to enhance the extraction of hot-electron carriers and lead to improved plasmoelectric conversion due to the strong coupling between Fabry-Perot nanocavity modes of the TiO 2 /Au and LSPR of AuNP. [34]In addition, Ng et al. showed for a 2D multilayer of Au/TiO 2 /Au configuration that the multistack layered Au configuration could result in broadband and near-unit light absorption. [35]Nevertheless, to the best of our knowledge, most related research work is mainly focused on thin film planar-coating design, which would give relatively low sensitivity thanks to the limited contact areas, making it not suitable for sensing applications.
In this work, a monolayer of Au-nanoparticle/TiO 2 -nanowire dandelion-like core-shell heterostructure (AuTNW-dCS) was fabricated, which would further self-assembled into a 2D arrayed plasmonic monolayer to form photoanode for photoelectric conversion.The AuTNW-dCS-based photoanode exhibits a remarkable broadband absorption in the visible spectrum from 500 to 700 nm and excellent photon-to-electron conversion performance by the reason of LSPR coupling effects of cross-linked AuNP monolayer and the efficient Schottky junction transfer in the Au/TiO 2 interface for facilitating the separation of photogenerated electrons and holes.With such promising optical and photoelectric properties, the as-prepared AuTNW-dCS can serve as a great candidate for visible-light-response photoanodes.Furthermore, the nanowired framework of TiO 2 can serve as a good biomolecule-loading site due to the high hydrophilic and biocompatible properties, which can enhance the analyte-binding efficiency and finally improve the sensing sensitivity.As a proof of concept, alpha-fetoprotein (AFP) has been robustly detected by this AuTNW-dCS based photoelectric immunosensor under visible LED light irradiation, with a wide detection range of 0.1-1000 ng mL −1 , low detection limit of 0.1 ng mL −1 , and excellent selectivity.It is worth noting that this proposed AuTNW-dCSbased 2D layer can give an impetus to develop a flexible sensing platform.Furthermore, our implementation involves only a simple process of sputtering deposition, thermal-alkaline treatment, and annealing, without the requirement of photopatterns, making it easy to fabricate in large-scale production at low cost.

Results and Discussion
A 2D plasmonic monolayer of assembly of Au-based nanoparticles is of scientific and technological interest due to their well-defined adjustable LSPR bands resulted from the intense light-matter interaction arising due to the plasmon confinement. [36,37]In this work, we designed a unique 2D plasmonic metal-semiconductor array composed of Aunanoparticle/TiO 2 -nanowire core-shell heterostructures directly onto the glass substrate through the combination of deposition process for Au nanoparticle growth and alkaline thermal treatment with subsequent annealing for TiO 2 nanowire fabrication.The synthetic process is similar to our previous work, with slight modifications. [21]Figure 1a-c and Figure S3 (Supporting Information) show the SEM and TEM images of the as-prepared Au-TiO 2 sample.It is clearly seen that a hierarchical TiO 2 @Au 2D hybrid array was successfully obtained through the assembly of the dandelion-like core-shell nanostructure (herein denoted as AuTNW-dCS), in which the TiO 2 nanowires (TNWs) with a thickness in the range of 10-50 nm was uniformly coated onto the surface of Au nanoparticles.The lattice fringes of the (111) and (101) crystal planes belonging to Au and anatase TiO 2 , respectively, can be clearly seen in Figure 1d,e, accompanied by an interplanar crystal spacing of ≈0.24 and 0.71 nm.Therefore, it can be concluded that the AuTNW-dCS was formed through the functional decoration of mesh-shell shaped TiO 2 nanowires onto the surface of the Au nanoparticles core.It is noted that the connection of adjacent AuNPs with cross-linked TiO 2 NWs was observed, which could build up a transfer path for photogenerated electrons, achieving faster electron transport and lower recombination (Figure 1f).Interestingly, this 2D AuTNW-dCS film can be easily transferred from the glass substrate into the tape by the peeling-off process, useful for the fabrication of flexible devices (Figure 1g).
Figure 2a plots the absorption spectrum of the as-prepared AuTNW-dCS monolayer as compared with those of two control samples: the bare TNW film and the AuNPs film.As expected, the bare TNW film has negligible absorption in the visible range.For the AuNPs film, an absorption peak at ≈525 nm with a fullwidth half-maximum (FWHM) of ≈41.2 nm is observed due to the LSPR excitation effect of plasmonic gold nanoparticles.For the AuTNW-dCS monolayer, it appears a significantly enhanced absorption in the visible spectrum of 500-700 nm, as well as a 75 nm redshift as compared with the AuNP without coating the TNW nanoshell.
][40][41][42] Such a broadband absorption behavior is primarily due to the fact of the formation of hot-electron carriers at the interfacial surface of the AuTNW-dCS nanostructures during the LSPR non-radiative decay, which is of great interests in science and technology. [27,35]The red-shift of the plasmon band after TNW nanoshell coating is caused by the strong hotelectron injection mechanism, a plasmon-induced charge separation at the Au-NP/TiO 2 interface, and the difference of the refractive index of the surrounding medium of AuNPs. [34,43]The higher the refractive index, the more redshift in the plasmon band.With the significant increase in the refractive index, air 1.00 and TiO 2 nanowires >1.50, [44] respectively, the large SPR wavelengthredshift is obtained.In addition, as compared to AuNP without TNW nanoshell coating, the peak is greatly intensified after the formation of the TNW shell onto it.It indicates the TiO 2 shell can serve as an effective reflecting layer to modify internal light reflection, which can greatly increase the opportunity of AuNP for photon absorption.The schematic mechanism of the non-radiative decay pathway is illustrated in Figure 2b.Initially, the d-band electrons of AuNPs follow the Fermi-Dirac distribution at the thermal equilibrium of the system.A highly strong plasmon-exciton coupling between the neighboring core-shell particles of arrayed AuTNW-dCS monolayer can be achieved.Thus, photo-excited with a LED visible-light source for the generation of increased  conversion results in the generation of measurable photocurrent.To study the efficiency of plasmon-induced hot-electron extraction for the AuTNW-dCS monolayers, the photocurrent responsibility is carried out under the on-off illumination of a 6 W LED visible light using a three-electrode test strip fabricated via screen printing technique (Figure 2b) in 1X phosphate-buffered saline (PBS) solution under zero external bias voltage.Figure 2c shows the photocurrent versus time (I-t) curve for the AuTNW-dCS photoanode and the AuNPs film.The present on/off ratio in AuTNW-dCS film is ≈23, whereas the AuNP film is relatively low, clearly indicating its potential for an opto-sensitive measuring device.As expected, a significant photocurrent enhancement is observed in the AuTNW-dCS photoanode, with a photocurrent density of 2.4 μA cm −2 , which is 17.7 times higher than that of the AuNP one.The photocurrent enhancement may result from the enriched visible light absorption and better incident-photon-tocurrent-carrier efficiency resulting from the enhanced separation of plasmonically generated hot electrons at the Au/TiO 2 interface.With an impressive on/off ratio and improved photoelectric activity, the AuTNW-dCS photoanode gives a new type of hybrid material that is promising for plasmon-based biosensor devices.
Further, a test strip photoelectric immunosensing device designed with three electrodes based on this AuTNW-dCS photoan-ode was used for sensitive and specific detection of the liver cancer biomarker AFP in human serum.The label-free immunosensor design was involved in only one-step immobilization process by directly assembling AFP-antibodies onto the TiO 2 nanoshell surface without the usage of surface bio-linkers as the recognition sites for a specific analyte because of the high hydrophilicity and excellent biocompatibility of Au-TiO 2 core-shells structure (top figure in Figure 3a). [45,46]Afterward, bioactive AFP antigens in human serum can be attached to the nanoshell surface conjugated with Anti-AFP due to the selective antibody-antigen recognition for AFP detection (bottom figure in Figure 3a).The presence of the Anti-AFP and AFP antigen could be confirmed by the analysis of electrochemical impedance spectroscopy (EIS).An increase of the charge-transfer resistance (R ct ), corresponding to the increase in the diameter of the semicircle, was observed upon immobilizing an antigen or antibody to the photoanode surface (Figure 3b), which would lead to the decrease of the photocurrents (Figure 3c).Therefore, the photocurrent change can be further used to quantitatively and qualitatively detect the AFP antigen with different concentrations.
The label-free detection of the AFP analyte was carried out by monitoring the photocurrent change at an applied potential of 0.0 V under LED visible light.concentrations of AFP antigen have an important influence on the photocurrent responses, where the photocurrent decreases accordingly with the AFP concentration increases.Figure 4b shows that from 0.1 to 1000 ng mL −1 , the change in photocurrent has a good linear relationship with the logarithm of the AFP concentrations with an R-square of 0.996, and a detection limit of 0.1 ng mL −1 at S/N ratio of 3.This may be due to the improved light harvesting by enhancing light absorption range from 2D Au nanoparticles film and high hydrophilicity and excellent biocompatibility for analysts loading from unique AuNP-TNW core-shells structure, allowing the designed immunosensor to have a wider detection range and a smaller detection limit.In addition to the sensor sensitivity, selectivity is also a significant indicator for successfully developing biosensor chips for clinical diagnosis.The selective determination of AFP antigen was assessed under the optimal conditions by measuring the photocurrent response change of AFP with concentrations of 0.1 and 1 ng mL −1 , compared with that of other samples, including PBS solution and 1000 ng mL −1 immunoglobulin G (IgG), as well as 1000 ng mL −1 hepatitis B virus (HBV) proteins.No significant change in photocurrent response was observed to the asassembled immunosensor with the additions of IgG and HBV proteins, suggesting its excellent selectivity.

Conclusion
In summary, a label-free photoelectric immunosensor using a 2D arrayed plasmonic monolayer composed of dandelion-like hierarchical metal-semiconductor (AuTNW-dCS) core-shell nanostructure as the photoactive material was successfully fabricated onto the glass substrate directly.The AuTNW-dCS-based photoanode displayed a sensitive photocurrent response for AFP detection in human serum under the LED light irradiation due to its better hot-electron carriers at interfacial surface for enhancing light harvesting and facilitating hot-electron transfer and good biocompatibility for analyst adsorption, leading to excellent sensing performances, including wide sensing range of 0.1-1000 ng mL −1 , low detection limit of 0.1 ng mL −1 , and promising selectivity.Furthermore, the proposed AuTNW-dCS-based 2D plasmonic array can be completely peeled off from the glass substrates, showing its potential for flexible photoanodes and promoting applications for soft or wearable optoelectronics.

Experimental Section
Preparation of Dandelion-Like Hierarchical Au-TiO 2 Core-Shell Nanostructure: The synthetic process was similar to the previous work, with slight modifications. [21]It involved two steps.i) Synthesis of AuNPs array: first, Au thin film with ≈3 nm in thickness was sputter-deposited onto the surface of a glass substrate through a magnetic sputter instrument (TVC-M8C8TV, Transvac co.Ltd).Then, rapid thermal annealing treatment under microwave condition was performed to form isolated AuNPs.The above processes were repeated for six times to obtain closely packed gold nanoparticle (AuNP) array with different particle sizes and ultrasmall gap sizes (Figure S1, Supporting Information), which induced optimal light absorption in the visible wavelength range and low electric resistance < 20 ohm sq −1 .ii) Formation of TNW onto AuNPs to form core-sell structure.The fabrication process is as follows.First, a 50-nm-thick Ti film was coated onto the AuNPs array by the sputtering-deposition method.Then sodium titanate (Na 2 TiO 3 ) NWs were fabricated through alkali-treatment by heat treating at 80 °C for 40 min in NaOH aqueous solution.Afterward, the sample was immersed in 0.1 m HNO 3 aqueous solution to replace Na + with H + .Then, the sample was rinsed with ethanol and dried under nitrogen gas flow.Finally, the obtained product was annealed at 500 °C to obtain dandelion-like hierarchical Au-TiO 2 core-shell nanostructure.The preparation process was shown in Figure S2 (Supporting Information).
Material Characterizations and Electrochemical Measurements: The morphology, structure, and composition of the prepared samples were investigated by field emission scanning electron microscope (FESEM, Zeiss Ultra Plus, accelerating voltage of 3 kV), and high-resolution transmission electron microscope (HRTEM, JEM 2010, 200 kV).UV-vis absorption spectra were collected on a UV-vis-NIR Spectrophotometer (SHIMADZU UV-3600 Plus).The electrochemical measurements were carried out on a CHI electrochemical workstation (CHI 611E, Texas Instruments Inc., Austin, TX, USA) that was connected to a three-electrodes test strip composed of the as-fabricated dandelion-like hierarchical Au-TiO 2 core-shell nanostructure for working electrode, conducting carbon as the counter electrode, and Ag/AgCl as the reference electrode.
Detection of AFP in Human Serum: For detection of AFP, an immunosensor was assembled by coating an anti-AFP antibody (My-BioSource, MBS530361) diluted in phosphate-buffered saline (PBS) solution onto the surface of the working electrode site of the above prepared three-electrode test strip with a simple drop-casting process.Next, a 20 μL AFP antigen with different concentrations in human serum which was prediluted 100-fold with PBS solution was dropped onto the immunosensors and incubated at ambient temperature for 30 min for analysis.After washing with PBS and drying under a stream of nitrogen, the electrode was used for photocurrent analysis by using the CHI 611E electrochemical workstation at an applied potential of 0.0 V in 1X PBS solution under 6 W visible LED-light illumination.

Figure 1 .
Figure 1.a,b) SEM image, c) TEM image, d,e) HRTEM images, and f) schematic diagram for the as-prepared 2D AuTNW-dCS based monolayer.g) Photo of the 2D AuTNW-dCS film peeling from the glass substrate.

Figure 2 .
Figure 2. a) Visible absorption spectra for the AuTNW-dCS monolayer, bare TNW, and AuNPs film.b) Schematic of non-radiative pathway mechanism for the AuTNW-dCS photoanode.c) Photocurrent response of the as-prepared photoanodes under zero external bias voltage, illuminated by a 6 W LED visible light.d) Schematic diagram of the three-electrode test strip in which the as-prepared sample is the working electrode (WE), Ag/AgCl is the reference electrode (RE), and carbon is the counter electrode (CE), respectively.
Figure 4a depicts the

Figure 3 .
Figure 3. a) Step-1: immunosensor assembly by the self-assembly of Anti-AFP directly onto the AuTNW-dCS photoanode, and step-2: the sensor was further used to incubated with AFP antigens.b) EIS Nyquist plot in the 0.5 mm [Fe(CN) 6 ] 3−/4− solution containing 0.1 m KCl and c) photocurrent response change analysis under visible LED light irradiation of (black line) AuTNW-dCS photoanode, (blue line) AuTNW-dCS conjugated with Anti-AFP, and (red line) AuTNW-dCS with Anti-AFP and AFP antigen.

Figure 4 .
Figure 4. a) Change in photocurrent response and b) the corresponding calibration curve based on the in-serum AFP antigen with different AFP concentrations.c) Photocurrent change of the immunosensor for 0.1 and 1 ng mL −1 AFP, 1000 ng mL −1 HBV, 1000 ng mL −1 IgG, and PBS solution.