## 1 Introduction  
Photodynamic therapy (PDT) is a light-activated chemotherapeutic treatment that utilizes singlet oxygen and reactive oxygen species to react with surrounding biological substrates, such as mitochondria, lipid membranes, nucleic acids, and proteins. The oxidative reaction either kills or irreversibly damages malignant cells. The process requires light at the appropriate wavelength to activate the photosensitizer (PS) accumulated in diseased tissues. Then the PS enters excited triplet states and interacts with ground-state triplet oxygen, yielding cytotoxic species, e.g., excited-state singlet oxygen or free radicals. Unfortunately, visible activation wavelengths cannot penetrate tissues deeply and seriously impede this therapeutic strategy for currently approved PSs.  
The step toward clinical practice prefers excitation with the least-invasive near-infrared (NIR) light, through which both blood and soft tissues have mild backscattering and low energy absorption, thus providing maximal tissue penetration. There are two approaches to generate singlet oxygen at the NIR wavelength. One primary method relies on chemical synthesis to yield PS with a large two-photon absorption cross-section [1; 2; 3; 4]. Another appealing strategy is taking fluorescence resonance energy transfer (FRET) as a tool to provide two-photon excitation of PS through the incorporation of a two-photon chromophore, e.g., dye molecules, acting as a donor to transfer energy for the excitation of a PS acceptor [5; 6; 7; 8]. Since noble metal nanostructures have shown promising two-photon luminescence by their surface plasmon resonance (SPR), [9; 10; 11; 12; 13; 14] the NIR-excited plasmons may serve as another kind of energy donor to excite PS. Herein, we present a platform exhibiting NIR PDT by taking advantage of the efficient nonlinear optical processes in core-shell gold nanopeanut structures [14]. The efficient two-photon SPR of gold nanopeanuts provides an energy source to excite PS for singlet oxygen generation. Importantly, the entire process can be triggered by the excitation of NIR wavelengths located in the second biological window (1000-1350 nm). Operating PDT at this wavelength regime offers better tissue penetration relative to 650-950 nm [15; 16]. For example, hemoglobin has much less absorption in the NIR wavelength, and blood is sufficiently transparent from 1000 to 1350 nm. Notably, this nanopeanut particle (length: 53 nm, width: 22 nm) with a size less than 100 nm exhibits two distinct NIR plasmon bands at approximately 1110 and 1300 nm. We employed a 1230 nm femtosecond (\(\sim\)100 fs) Cr: forsterite laser to resonantly excite the surface plasmons and yield efficient two-photon luminescence of nanopeanuts. Meanwhile, commercial toluidine blue O (TBO) was conjugated on the surface of the nanopeanut. With a spectral overlap of SPR and TBO absorption bands at 600-650 nm, NIR-excited SPR can enhance two-photon absorption and transfer the excited charges to TBO PS to generate singlet oxygen (Figure 1). This new NIR PDT nanostructure also presents a nonlinear optical imaging capability to serve as a theranostic agent. Nonlinear optical imaging is known as an attractive technique for in vitro and in vivo biological applications because of its superior capability of deep-tissue observation. In this study, we found that the SPR effect resulted in improved two-photon fluorescence from TBO-conjugated nanopeanuts, giving stronger imaging contrast than nanopeanuts alone (no TBO conjugation) and making it a more promising theranostic agent in deep tissue.