Near-infrared II nanomaterials for hypoxic TME diagnosis and therapy

ABSTRACT

Hypoxic tumor microenvironments (TME) drive cancer aggression and therapy resistance, necessitating precise diagnostic and therapeutic strategies. Near-Infrared II (NIR-II) nanomaterials have emerged as transformative tools for addressing hypoxic TME challenges, leveraging deeper tissue penetration, minimal scattering, and reduced autofluorescence. This review systematically summarizes the recent advancements in NIR-II nanomaterials, focusing on their applications in imaging and therapy for hypoxic tumors. NIR-II-based imaging techniques, including fluorescence, photoacoustic, and multimodal imaging, provide high sensitivity and resolution for detecting hypoxic regions, aiding in early diagnosis and personalized treatment planning. In therapeutic applications, NIR-II materials enhance the efficacy of photothermal therapy, photodynamic therapy, and photo-Fenton therapy, all of which overcome the limitations of traditional therapies in hypoxic areas. The combination of NIR-II materials with other treatment modalities holds promise for more effective cancer treatments. This review also discusses the challenges in the current research landscape, including the preparation, scalability, and clinical translation of NIR-II materials. It highlights the need for continued innovation to optimize these materials for broader clinical applications. Overall, NIR-II nanomaterials represent a significant step forward in addressing the challenges posed by hypoxic tumors, with the potential to revolutionize cancer diagnosis and therapy.

TRANS-ABSTRACT

NIR-II nanomaterials offer breakthrough potential in hypoxic tumor diagnosis and therapy. With deep tissue imaging, reduced scattering, and hypoxia-targeted treatments (photothermal, photodynamic, photo-Fenton), they combat treatment resistance. Promising for precision medicine!

GRAPHICAL ABSTRACT

Summary of engineered NIR-II nanomaterials in hypoxic TME modulation: from diagnostic imaging to targeted therapy.

1. Introduction

The tumor microenvironment (TME) is a highly complex and dynamic system that plays a critical role in cancer progression and treatment resistance [1,2]. One of the key features of TME is hypoxia, a condition where the supply of oxygen is insufficient to meet the demands of rapidly proliferating tumor cells [3,4]. Hypoxia creates a hostile microenvironment that not only drives tumorigenesis but also contributes significantly to the resistance of tumors to conventional treatments such as chemotherapy and radiotherapy [5,6]. Hypoxic TME promotes a range of malignant behaviors, including enhanced invasion, metastasis, and immune evasion, making it a central target for cancer therapy [7]. Therefore, accurate diagnosis and effective modulation of the hypoxic microenvironment are essential for improving cancer treatment outcomes [8]. Investigating and addressing hypoxia within the TME offers a promising strategy for enhancing the efficacy of current therapies and developing new therapeutic approaches for cancer.

Near-infrared-II (NIR-II) nanomaterials refer to functional materials exhibiting optical responses (absorption/emission) within the 1000 – 1700 nm wavelength range [9]. Classified by composition and structure, they primarily comprise [10]: (1) Organic nanomaterials, including small-molecule dyes (e.g., D-A-D structured dyes), conjugated polymers (e.g., semiconducting polymer nanoparticles, SPNs), and aggregation-induced emission luminogens (AIEgens). (2) Inorganic nanomaterials, such as rare-earth nanoparticles (RENPs), quantum dots (QDs), carbon-based materials, and noble metal nanoclusters. Compared to conventional Near-Infrared I (NIR-I) (with a region of about 700 to 1000 nm) materials, NIR-II materials offer several advantages, including deeper tissue penetration, reduced light scattering, and low autofluorescence [11]. These characteristics make NIR-II materials particularly suitable for biomedical applications, especially in imaging and diagnosing the hypoxic tumor microenvironment [10,12]. The increasing body of research on NIR-II and its applications in hypoxic TME, as evidenced by the growing number of studies published each year, highlights the expanding interest and potential of these materials. Based on an analysis of the Web of Science search results, it is evident that the number of academic papers related to the topic of hypoxia has remained relatively stable over the past decade (Figure S1(a,b)). In contrast, there has been a notable increase in the number of publications on NIR-II, and more interestingly, the growth trend of NIR-II and hypoxia papers mirrors this increase (Figure S1(c)). These trends suggest that the application of NIR-II nanomaterials in the hypoxic tumor microenvironment (TME) is becoming more prominent. With the continuous emergence of new research each year in this area, it is crucial to systematically review and summarize the existing findings and address the challenges that researchers are currently facing. This will not only help scientists gain a deeper understanding of this rapidly evolving field but also guide future investigations. Given the breadth and depth of these findings, it is essential to systematically review and summarize the current research to better guide the future application of NIR-II nanomaterials for hypoxic TME diagnosis and therapeutic intervention.

Current research on Near-Infrared II (NIR-II) materials in the hypoxic tumor microenvironment (TME) is focused on two key aspects: diagnostic and therapeutic applications. In diagnostics, NIR-II imaging proves effective in identifying hypoxic regions within tumors, crucial for accurate diagnosis and treatment planning. Fluorescence imaging utilizes NIR-II materials’ ability to selectively bind to hypoxic areas, providing high sensitivity for detecting deep-seated tumor hypoxia, which conventional techniques may miss [13]. Photoacoustic imaging combines optical and acoustic techniques, enhancing the detection of hypoxic tumors with high-resolution, real-time visualization [14,15]. Multimodal imaging integrates NIR-II fluorescence, photoacoustic imaging, and other modalities like MRI and CT, offering a comprehensive tumor assessment, combining structural and functional insights for early diagnosis and personalized treatments [16–19]. Therapeutically, NIR-II materials help overcome challenges posed by the hypoxic TME, where traditional therapies often fail [10,20]. Photothermal therapy (PTT) uses NIR-II materials to absorb NIR light and generate heat, selectively destroying tumor cells in hypoxic regions with deep tissue penetration [21,22]. Photodynamic therapy (PDT) leverages NIR-II materials to improve the efficacy of conventional PDT by enhancing light penetration and targeting photosensitizers in hypoxic tissues [23–26]. Photo-Fenton therapy utilizes NIR-II materials to catalyze the Fenton reaction, generating reactive hydroxyl radicals that can effectively kill tumor cells under low-oxygen conditions [27,28]. Additionally, combination therapies involving NIR-II materials, such as PTT with chemotherapy or PDT with immunotherapy, aim to address tumor resistance by employing multiple treatment mechanisms [29,30]. This comprehensive approach enhances overall treatment efficacy, providing promising strategies for tackling hypoxic tumors and improving cancer treatment outcomes.

Given the growing research interest in NIR-II nanomaterials, their application in the hypoxic tumor microenvironment is an exciting and rapidly evolving field. With significant advantages in deep-tissue imaging, targeted therapy, and multimodal treatment approaches, NIR-II materials provide new opportunities for accurate diagnosis and effective treatment of hypoxic tumors. This review highlights the current research landscape and the potential of NIR-II materials to revolutionize cancer diagnostics and therapy. By summarizing recent advancements, we aim to provide valuable insights for the future design, optimization, and clinical translation of NIR-II nanomaterials. Continued research in this area will be essential for further improving cancer treatment outcomes and enhancing the clinical applicability of hypoxia-targeted therapies.

2. The overview of NIR-II nanomaterials for imaging and treatment hypoxic regions

Compared to NIR-I (with a region of about 700 to 1000 nm) [9,31], the NIR-II spectrum presents several advantages: (1) The light waves in the NIR-II spectrum can permeate biological tissues more effectively, making them suitable for imaging deep structures [32]. (2) Due to their longer wavelength, NIR-II light undergoes relatively reduced scattering and absorption in tissues, leading to diminished background noise during imaging [33]. This enhancement augments both the clarity and contrast of images. (3) With its extended wavelength, the NIR-II region can provide higher spatial resolution, permitting imaging at more detailed scales and enabling lucid visualization of the minute structures within biological entities [34]. (4) NIR-II light inflicts less damage on biological systems, rendering it suitable for in vivo imaging and long-term observation. Overall, compared to NIR-I, NIR-II materials display deeper tissue penetration, diminished background noise, heightened resolution, and excellent biocompatibility [35].

NIR-II nanomaterials have shown great potential in diagnosing oxygen deficiency, particularly in cancer research, due to their unique optical properties in the near-infrared region. These materials are commonly used for imaging hypoxic regions in tumors, offering deep tissue penetration and high-resolution imaging. NIR-II fluorescence nanomaterials, such as rare-earth metal-doped nanoparticles, carbon quantum dots, and semiconductor quantum dots, are employed to visualize hypoxic areas accurately, aiding in the early detection and precise monitoring of tumors [36–40]. Additionally, NIR-II photoacoustic nanomaterials, including gold-silver alloys and graphene derivatives, combine both optical and acoustic signals, providing enhanced deep tissue imaging capabilities [41,42]. This allows for better diagnosis of oxygen-deficient tissues and improves our understanding of hypoxia in disease progression [43,44].

In terms of treatment, NIR-II nanomaterials offer promising solutions for therapy, especially for oxygen-deficient regions, such as tumors. NIR-II photothermal nanomaterials, like gold nanoparticles, copper sulfide nanoparticles, and carbon-based materials (e.g., graphene oxide, carbon nanotubes), can convert NIR-II light into heat, making them ideal for photothermal therapy (PTT) [45,46]. This thermal effect can effectively target and treat hypoxic tumor tissues [47,48]. NIR-II photo-Fenton nanomaterials utilize NIR-II light-activated Fenton reactions with metal ions and hydrogen peroxide to generate hydroxyl radicals, offering dual therapeutic effects for tumor treatment, especially in hypoxic areas, by combining photodynamic therapy and Fenton reactions [49,50]. Moreover, the NIR-II light responsive drug delivery system, which are typically designed with light-sensitive materials or links between the drug and nanoparticle that undergo a structural change when exposed to NIR-II light. By targeting oxygen-deficient tissues, these materials improve the efficacy of therapeutic agents. The combination of imaging and therapeutic properties of NIR-II nanomaterials opens new possibilities for precision medicine, allowing for more targeted and effective treatments of hypoxia-related diseases [51,52].

3. The application of NIR-II materials in hypoxic TME imaging

3.1. NIR-II-based tumor imaging technology

NIR-II imaging technology has become a pivotal method for tumor imaging, due to its exceptional depth penetration capacity and low background signal. This technology largely comprises two techniques: NIR-II fluorescence imaging and photoacoustic imaging. NIR-II fluorescence imaging employs near-infrared light to excite fluorescent probes, thus allowing high-sensitivity imaging of tumor cells [13,21,22,24,25,53]. The decrease in scattering of fluorescence signals within tissues permits NIR-II fluorescence imaging to render clear images at substantial depths within the tissue. Furthermore, NIR-II photoacoustic imaging harnesses the benefits of both optical imaging and ultrasound imaging. When near-infrared light is directed at the tumor tissue, the absorbent substances within it (such as hemoglobin) transfigure the absorbed light energy into heat, thereby engendering ultrasound signals. These signals can then be recorded and employed to reconstruct information about the tumor’s structure and blood flow. Pairing NIR-II fluorescence imaging with photoacoustic imaging has the potential to greatly refine the precision and comprehensive nature of tumor imaging. The mutual enhancement of both modalities allows for the concurrent provision of information pertaining to the tumor’s morphology, structure, and physiological status, thereby enabling physicians to garner a more holistic understanding of tumor characteristics during diagnosis and treatment [44,46,54,55]. This multimodal imaging strategy not only enhances diagnostic precision but also provides powerful tools for tumor monitoring and personalized therapy.

3.2. The application of NIR-II fluorescence imaging in hypoxic TME detection

The hypoxic state embedded within the tumor microenvironment is a significant driving force for tumor progression and metastasis. NIR-II fluorescence imaging, by harnessing specific fluorescent probes, empowers real-time surveillance of oxygen levels in tumor tissues. These probes typically possess high sensitivity to oxygen, and their fluorescence signals undergo pronounced alterations when oxygen concentrations dwindle, enabling the visualization of hypoxic zones [21,22,25]. This capability facilitates physicians to swiftly pinpoint hypoxic regions within tumors, laying the groundwork for personalized treatment strategies. Furthermore, the superior resolution of NIR-II fluorescence imaging paves the way for dynamic observation of changes within the tumor microenvironment, delivering novel instruments for the exploration of tumor biology. Li et al., 2021, designed and synthesized an anoxic response probe named TBTO containing four diethylamine n-oxide groups. TBTO can be converted to TBT in an anoxic microenvironment by reductases such as CYP450, which has a typical D-A-D structure and strong TICT effect. TBTO NPs showed sustained near-infrared fluorescence and photoacoustic height signals in hypoxic cells and tumor tissues. This research results provides a promising new method for clinical cancer diagnosis [56] (Figure 1).

Figure 1.

Hypoxia-activated NIR-II fluorescent probe for tumor imaging. (a) Schematic illustration of hypoxia-activated probe for NIR fluorescence and photoacoustic dual-mode tumor imaging. (b) Fluorescence responses of TBTO NPs to different culture conditions (normoxic or hypoxic) of HeLa cells. (c) In vivo PA images of the tumor site in the mice after tail intravenous injection with TBTO NPs (200 mL, 1 mg/mL) or saline (200 mL) as control. Scale bar represents 2 mm. Reproduced from [56] under the terms of a creative commons attribution license (https://creativecommons.org/licenses/by/4.0/).

3.3. Applications of NIR-II photoacoustic imaging in monitoring blood oxygen saturation

NIR-II photoacoustic imaging serves as a vital tool in overseeing blood oxygen saturation. By harnessing lasers of specific wavelengths, photoacoustic imaging can accurately determine tissue oxygen saturation based on the absorption traits of oxygenated and deoxygenated hemoglobin in the blood. This technology boasts exceptional spatial resolution and deep tissue penetration, enabling real-time monitoring of oxygen status in the vicinity and interior of tumors [14,15,21,24,56,57]. This capability proves critical for assessing the physiological condition of tumors and gauging treatment efficacy, supplying clinicians with a more comprehensive biomarker monitoring. Zeng et al. developed a biomimetic oxygen delivery system called BLICP@O₂, which utilizes tumor cell membranes and heat-sensitive liposomes as oxygen carriers, combined with the NIR-II dye IR1048, the photosensitizer chlorophyll e6 chloride (Ce6), and perfluorohexane. Through sequential irradiation at 1064 and 690 nm, BLICP@O₂ showed significant photothermal and photodynamic effects. Blood oxygen changes during PDT could be tracked using multispectral photoacoustic imaging. This study proposes an image-guided, dual-wavelength programmed cascade therapy strategy for tumor-targeted oxygen delivery and controlled release, with real-time monitoring of efficacy via photoacoustic imaging [24] (Figure 2).

Figure 2.

Biomimetic oxygen delivery system for blood oxygen saturation monitoring. (a) Schematic illustration of the synthesis of BLICP@O₂. (b) Changes in O₂ concentration in BLICP@O₂ and BLICP solutions under different treatments. (c) Representative 2D PA images of solid tumors showing a parametric map of estimated oxygen saturation (sO₂) (scale bar = 0.5 cm). D. Quantification of photoacoustic blood oxygen in (C) (mean ± SD, n = 3). Reproduced with permission from [24]. Copyright 2023, Wiley.

3.4. The synergistic effect of NIR-II imaging with other diagnostic techniques

NIR-II imaging technology effectively supplements traditional imaging modalities, like MRI and PET. NIR-II imaging delivers high-resolution tumor imaging that divulges the microscopic structure and hypoxic conditions of tumors, whereas MRI provides a more extensive panorama of tissue information and functional attributes, and PET evaluates the metabolic status of tumors. By amalgamating these imaging techniques, physicians can garner a more comprehensive perspective of tumor information, thereby informing treatment decisions. Moreover, NIR-II imaging can be conducted prior to MRI or PET imaging to help finesse the imaging protocol, enhancing both diagnostic precision and efficiency. Lyu et al. developed an ultra-smallcysteamine (Cys) coated FePd bimetallic nanodots (NDs) that were found to effectively induce hyperthermia when irradiated in the NIR-II region (1064 nm). These NDs were observed to accelerate tumor blood flow, increase oxygenated hemoglobin, alleviate hypoxia, and enhance sensitivity to radiotherapy. The iron component in these NDs contributes to MRI contrast enhancement, while palladium improves CT imaging. confirmed the enrichment of NDs at the tumor site, with significantly boosted CT/MRI/PA signals, offering a comprehensive solution for the precise diagnosis and treatment of solid tumors like breast cancer [16]. (Figure 3) These multi-modal imaging approach facilitates a more accurate tumor diagnosis and management (Table 1).

Figure 3.

Bimetallic nanodots for three-mode CT/MRI/PA imaging. (a) The process of NDs synthesis and its application for synergistic thermoradiotherapy and tri-modal imaging. (b) Representative immunofluorescence images of DAPI, anti-CD31 antibody and antipimonidazole for tumor slices. (c) The blood oxygen saturation before, 24 h after solution and 30 min, 1 h, 6 h and 12 h after laser irradiation under 1064 nm irradiation at 1 W/cm². D. in vivo PA images of tumorous sites of mice before and after i.V. injection of NDs solution. E. in vivo CT images of tumorous sites of mice before and after i.V. injection of NDs solution. F. in vivo T2-weighed images of tumorous sites of mice before and after i.V. injection of NDs solution. Reproduced with permission from [16]. Copyright 2020 Elsevier.

Table 1.

Imaging resolution/performance of different NIR-II materials.

NIR-II material	Imaging resolution/performance
BLICP@O₂ Nano-platform [24]	>2 cm PA imaging penetration depth, enabling real-time oxygen saturation monitoring
Z-GGRFF-IR775 Probe [54]	4.7-fold PA signal enhancement, significantly improved fluorescence signal-to-noise ratio
Ir-NP + COF@T [71]	Ratiometric hypoxia quantification (R2 = 0.994), >5 mm tissue penetration depth
Au@Pt-DOX-PCM-PEG [65]	41.3% NIR-II photothermal efficiency, tri-modal CT/MRI/PA imaging
BTZ/Fe2 + @BTF/ALD [67]	>5 mm NIR-II penetration depth, 8.94-fold bone-targeted enrichment enhancement
LSCaFPCe6 [25]	>4 mm NIR-II fluorescence penetration depth (porcine tissue model)
PBNP@NKs [70]	66.89% NIR-II photothermal conversion efficiency, superior penetration depth vs. NIR-I
FePd NDs [16]	CT attenuation coefficient: 24.8 HU·mL/mg; MRI relaxivity: r₂ = 30.7 mM−1 s− 1
TBTO [22]	Dual-mode NIR-I fluorescence (740 nm) and PA imaging; 4.1-fold hypoxia-activated signal enhancement
APCZ [26]	PA signal peak at 10 h post-injection, >3 mm penetration depth
Y6-Th [64]	>5 mm NIR-II penetration depth; quantum yield (ΦF): 1.9%
Lip(PTQ/GA/AIPH) [21]	Dual-mode NIR-II fluorescence/PA imaging; >5 mm penetration depth
TAT-Pd@Au/Ce6/PAH/H-MnO₂ [62]	56.9% photothermal efficiency; NIR-II imaging-guided nucleus targeting
bio-Ag₂Se-CAT QDs [57]	10-fold higher PA signal intensity vs. ICG; 75.3% photothermal efficiency

4. Applications of NIR-II nanomaterials in hypoxic TME treatment

NIR-II nanomaterials have shown great potential in the treatment of hypoxic tumor TME. NIR-II PTT utilizes the efficient thermal conversion of NIR light to precisely kill tumor cells while minimizing damage to healthy tissue. NIR-II PDT can activate reactive oxygen species (ROS) in low-oxygen environments, addressing the hypoxia-related limitations of traditional PDT. NIR-II nanomaterials can also be used for targeted drug delivery, enhancing drug release efficiency through stimulus-responsive mechanisms, and improving tumor penetration and retention. Furthermore, NIR-II nanomaterials offer advantages in multifunctional theranostics, enabling the synergistic multiple therapeutic modalities, providing a comprehensive solution for cancer treatment in hypoxic TMEs [58–60] (Table 2).

Table 2.

Antitumor efficacy of different NIR-II materials.

NIR-II material	Antitumor efficacy (In Vitro)	Antitumor efficacy (In Vivo)
BLICP@O₂ [24]	60% apoptosis rate under dual-laser irradiation	80% tumor volume reduction, HIF-1α downregulation
Z-GGRFF-IR775 [54]	PA signal enhancement: 4.7×, 99.4% cell viability at 10 μM	2.9× PA signal enhancement in tumors at 2 h post-injection
Ir-NP + COF@T [71]	4× higher cell death in hypoxia vs. normoxia	>90% tumor suppression with multiple irradiations
Au@Pt-DOX-PCM-PEG [65]	81.2% cell death (PTT+chemotherapy+H₂O₂)	>90% tumor volume reduction, complete regression in some cases
BTZ/Fe2 + @BTF/ALD [67]	>95% cell death under laser irradiation	100% tumor eradication at 14 days
LSCaFPCe6 [25]	>95% cell death (pH 5.5 + laser)	100% tumor elimination in bone metastasis model at 18 days
PBNP@NKs [70]	>95% cytotoxicity under laser	100% tumor suppression, 12.76× weight reduction vs. control
FePd NDs [16]	64.8% cell death (PTT+radiotherapy)	97.5% tumor volume inhibition
TBTO [22]	50% fluorescence reduction in hypoxia	Tumor PA signal peak at 6 h post-injection
APCZ [26]	% cell viability at 200 μg/mL	Complete tumor ablation at 14 days
Y6-Th [64]	IC₅₀ = 41.35 μg/mL in hypoxia	Complete tumor suppression at 14 days
Lip(PTQ/GA/AIPH) [21]	>95% cell death (triple therapy)	Complete tumor inhibition at 14 days
TAT-Pd@Au/Ce6/PAH/H-MnO₂ [62]	4.67% cell viability (PTT+PDT)	96.8% tumor volume reduction at 14 days
bio-Ag₂Se-CAT QDs [57]	>90% cell death under 808 nm laser	100% tumor eradication at 16 days

4.1. NIR-II photothermal therapy (PTT)

NIR-II photothermal therapy harnesses materials from the second near-infrared window, such as gold nanoparticles and carbon nanotubes, to absorb NIR-II light and convert it into thermal energy, thus inducing localized heating of tumor cells and leading to thermal ablation [29,61,62]. Owing to the low scattering and absorption traits of the NIR-II wavelength, this therapy can effectively permeate tissues, facilitating deep tissue penetration [21,63]. Furthermore, photothermal materials can be steered toward tumor cells, reducing harm to neighboring healthy tissues and conferring a selective advantage [22,55,57,64]. Zhang et al. developed a novel multifunctional nanoplatform, HAPsDH, which loaded DOX onto hollow gold-platinum bimetallic nanoshells and was modified with hyaluronic acid. Platinum nanoparticles catalyze the decomposition of tumor endogenous H₂O₂ into O₂, under hypoxic conditions, HAPsDH combined with laser significantly inhibited the viability of 4T1 cells [65]. We further analyzed the association between the photothermal conversion efficiency of different materials and the tumor ablation effect (Table 3).

Table 3.

The photothermal conversion efficiency (PCE) and the tumor ablation effect of different materials.

NIR-II materials	PCE	Tumor ablation effect
HAPsDH [65]	30.2%	79.3% (in vitro)
Au@Pt-DOX-PCM-PEG [29]	41.3%	81.2% (in vitro)
IR1048-MZ [22]	20.2%	Tumor completely ablated in 5 days (in vivo)
Lip(PTQ/GA/AIPH) [21]	42.15%	91.3% (in vitro)
Ag(2)Se Quantum Dots [57]	51.7%- 75.3%	>90% (in vitro)
Y6-Th NPs [64]	75.8%	>80% (in vitro)

4.2. NIR-II photodynamic therapy (PDT)

Photodynamic therapy harnesses NIR-II-excited photosensitizers that react with oxygen in tumor cells to generate singlet oxygen and free radicals, resulting in cellular damage or demise. Although this method usually works best in oxygen-rich conditions, making it oxygen-dependent, NIR-II materials can be purposefully designed to enhance their activity in hypoxic conditions [62,64]. The BLICP@O₂ nanosystem synergistically kills hepatocellular carcinoma cells via a dual-wavelength excitation cascade strategy [24]. The 1064 nm laser triggers the photothermal effect of BLICP@O₂, inducing liposome rupture and thereby releasing oxygen. Following the increase in oxygen concentration, the 690 nm laser activates chlorin e6 (Ce6) to generate substantially more singlet oxygen (¹O₂), resulting in significantly enhanced ROS generation. This approach demonstrates that photothermal therapy (PTT) enhances the ROS-mediated cytotoxic efficacy of PDT, offering an effective strategy to overcome hypoxia limitations in PDT [24]. (Figure 4) Moreover, The LSCaFPCe6 nanoplatform synergistically integrates O₂ self-supply, calcium overload, and NIR-II imaging-guided precision to potentiate PDT efficacy, suppress tumor growth, and minimize systemic toxicity, offering a promising strategy for hypoxia-resistant cancer therapy [25].

Figure 4.

BLICP@O2 nanosystems enhancing PDT for HCC in vitro. (a) Schematic of in vitro precision PDT for HCC. (b) Live/dead staining of Huh7 cells after different treatments. CCe6 = 0.5 mg mL − 1. Viable cells (green) were stained with calcein-AM, and dead cells (red) were stained with PI (scale bar = 500 μm). Reproduced with permission from [24]. Copyright 2023, Wiley.

4.3. NIR-II-driven chemodynamic therapy

By utilizing NIR-II light to instigate specific chemical reactions, such as facilitating drug release or boosting drug activity, we can accomplish effective tumor destruction [66–68]. This therapeutic approach can be tailored to release drugs at precise wavelengths, thereby enhancing drug localization and utilization, while also enabling a controlled release mechanism. Moreover, selective activation can augment biocompatibility and mitigate the side effects associated with the medication. For instance, Zhang et al. designed a novel radical nanogenerator, APCZ (AIPH/PDA@CuS/ZIF-8), which thermally triggers the decomposition of the radical initiator AIPH under NIR-II laser (1064 nm). APCZ in combination with laser significantly induced apoptosis of 4T1 cells under hypoxic conditions [26]. (Figure 5).

Figure 5.

Efficient synergistic treatment of hypoxic tumor by novel free radical nanogenerator. (a) Schematic illustration of the preparation of APCZ. (b) Cell viabilities of 4T1 cells after different treatments in normoxic and hypoxic conditions. Data shown as mean±SD, n = 3 per treatment. Statistical significance was set at *p < 0.05, **p < 0.01, ***p < 0.001. C. The calcein AM/PI co-stained images of 4T1 cells after different treatments in both normoxic and hypoxic conditions. Scale bar = 50 μm. Reproduced from [26] under the terms of a creative commons attribution license (https://creativecommons.Org/licenses/by/4.0/).

4.4. Combination therapy of NIR-II materials with hypoxia-activated drugs

By integrating NIR-II materials with drugs that activate in hypoxic environments, NIR-II photothermal or photodynamic effects can be leveraged to initiate hypoxia-activated drugs, thereby augmenting their efficacy within the tumor microenvironment and prompting a synergistic effect [69]. Wang et al., designed a new photothermal polymer TT-BDT by introducing strong electron-absorbing trifluoromethyl groups, and then co-assembled with hypoxia-responsive prodrug pro-PTX into nanoparticles (PBNPs), which were coated with natural killer (NK) cell membranes and endowed tumor targeting. PBNP@NKs has the best killing effect on 4T1 breast cancer cells under hypoxia [70]. Moreover, targeted delivery of NIR-II materials allows for a more thorough intervention in the tumor microenvironment [71].

5. Biocompatibility and metabolism issues of NIR-II materials

5.1. The biocompatibility of NIR-II materials

Gold nanoparticles (AuNPs) effectively reduce cytotoxicity and present good cell compatibility upon proper surface modifications [19]. Their stability in biological environments is further bolstered by modifications with polymers like polyvinyl alcohol (PVA) [72,73]. Additionally, AuNPs have been shown to modulate the activity of dendritic cells [74], enhance the function of macrophages, and yet, not provoke significant immune responses [75]. Functionalized carbon nanotubes (CNTs) show commendable biocompatibility in cell culture [76]. By modifying the surface functional groups, their cytotoxicity can be minimized, and cellular uptake efficiency can be maximized. While moderate amounts of CNTs do not cause significant toxic responses in vivo, the need for further research to investigate long-term effects remains [77]. Graphene and its oxides (graphene oxide, GO) show low cytotoxicity and in some cases, promote cell proliferation [78,79]. Research has also shown that GO can modulate the immune system’s response by reducing the release of inflammatory factors, thus improving biocompatibility [79]. Post surface modification, NIR-II quantum dots (QDs) demonstrate superior performance in biological imaging and significantly reduced cytotoxicity [37]. Although some QDs may induce minor toxicity in vivo, appropriate ligand modification can enhance their biocompatibility and biodegradability [38]. Polymer nanoparticles, including polylactic acid and polyvinyl alcohol, are earning recognition due to their excellent biocompatibility and biodegradability. They’ve shown good biocompatibility in both cell culture and animal models. Incorporating bioactive molecules, like peptides and antibodies, can enhance their targeting prowess while maintaining biocompatibility [80]. Metal sulfides such as CuS and Ag2S display exceptional performance in NIR-II imaging [19]. Upon proper functionalization, they present lower cytotoxicity [77]. Studies using mouse models indicate that the in vivo application of materials like CuS does not result in significant biotoxicity, enabling effective imaging [45]. In summary, with the proper design and surface modification, different types of NIR-II materials can achieve efficient bioimaging and other biomedical applications while maintaining good biocompatibility.

5.2. The metabolic and clearance mechanisms of NIR-II materials in vivo

The metabolic and clearance mechanisms of NIR-II materials in vivo are critical factors ensuring their safety and effectiveness. Their distribution within biological systems depends on their physicochemical properties, such as particle size, surface charge, and hydrophilicity. Some materials tend to build up in specific tissues, for example, the liver, spleen, and lungs, while others may concentrate within tumor tissues. NIR-II materials can achieve targeted delivery through surface modification, enhancing their accumulation in particular tissues or cells. This targeting not only improves the contrast of imaging but can also impact their metabolic and clearance mechanisms. Multiple NIR-II materials, such as specific polymer nanoparticles and metal sulfides, can degrade in vivo via enzymatic, oxidative, or hydrolytic processes. This leads to the generation of nontoxic, biocompatible small molecules that are eventually absorbed or excreted by cells. Smaller NIR-II materials, generally less than 5–10 nanometers in diameter, can be filtered by the kidneys and excreted in urine [81,82]. This serves as the primary clearance pathway for various nanoparticles, including quantum dots and gold nanoparticles. Larger nanoparticles are typically trapped by macrophages in the mononuclear phagocyte system, such as the liver, spleen, and lungs, and are processed via intracellular degradation [83]. (Figure S2) They are ultimately excreted as metabolic products. The metabolic and clearance mechanisms of NIR-II materials also come under the influence of the immune system. Certain materials may initiate acute or chronic inflammatory responses, which can subsequently affect their metabolism and clearance in the body. For materials remaining in the body for extended periods (like certain polymers or metal nanoparticles), it is essential to carry out long-term toxicity and biocompatibility assessments. These evaluations ensure that these materials do not accumulate and trigger chronic toxicity.

5.3. Long-term toxicity assessment of NIR-II materials

In the context of long-term use, NIR-II materials can have several potential toxic effects. Upon exposure to NIR-II materials, cells and tissues can exhibit multiple adverse effects. Quantum dots, for instance, can cause cellular oxidative stress, which can lead to DNA damage, lipid peroxidation, and protein denaturation [84]. Similarly, metal nanoparticles like gold nanoparticles (AuNPs) can interfere with cellular functions by binding to proteins and altering their structures [85]. Polymer-based NIR-II materials, while generally considered safer, can cause cellular toxicity through physical interaction or entrapment within cells [86]. Chronic exposure to NIR-II materials can instigate inflammatory responses in tissues. For example, quantum dots can activate the immune system, leading to the release of pro-inflammatory cytokines [87]. Similarly, AuNPs can elicit an immune response by activating macrophages and stimulating the production of reactive oxygen species [88]. Polymer-based NIR-II materials, depending on their size and surface characteristics, can also trigger inflammation [89]. In vivo, NIR-II materials can accumulate in various organs, leading to potential long-term effects. Quantum dots, due to their size, can accumulate in the liver and spleen [90]. AuNPs tend to deposit in the liver and kidneys, where they can cause damage over time [91]. Polymer-based NIR-II materials, due to their larger size and hydrophilic nature, can accumulate in the lungs, liver, and spleen [92].

6. Future perspective

NIR-II materials, due to their exceptional optical properties, have emerged as a promising tool for imaging and therapy, particularly for targeting hypoxic tumor microenvironments (TMEs). These materials exhibit superior tissue penetration, reduced background noise, and enhanced resolution compared to traditional NIR-I materials, making them valuable for biomedical applications. They have been applied in various imaging techniques such as fluorescence and photoacoustic imaging, as well as in therapeutic strategies like photothermal, photodynamic, and chemokinetic therapies. Despite these advancements, several limitations persist. The preparation processes for NIR-II materials are intricate and resource-intensive, often requiring rare elements or complex precursors that increase costs. Furthermore, the scalability of these materials remains a significant challenge, with issues related to the stability of the production process during mass manufacturing and the lack of environmental sustainability in the synthesis processes.

Furthermore, while the NIL-II material presents a prospect for clinical application, there are still several obstacles to its widespread clinical use. Such as toxicity risks from long-term organ retention (e.g., RES accumulation of RENPs > 30% ID/g) and immune activation (e.g., AuNP-induced macrophage ROS). The scalability limitations due to rare-element dependency (e.g., Yb/Pd, cost >$500/g) and batch inconsistency (>20% size variation in core-shell synthesis). The targeting inefficiency with tumor accumulation < 10% ID/g and hypoxia-compromised therapy efficacy (e.g., traditional PDT efficiency < 80% in hypoxic TME). And clinical integration hurdles including high-power laser thermal risks (>1 W/cm²) and lack of regulatory standards. To overcome clinical translation barriers, NIR-II nanomaterials require: (1) Renal-clearable ultrasmall probes (HD < 6 nms) and enzyme-degradable polymers (e.g., MMP-responsive SPNs fragmenting to <1 nm) to minimize organ retention. (2) Earth-abundant alternatives (e.g., CuS NPs with > 35% photothermal efficiency) and AI-guided microfluidic synthesis ensuring batch uniformity (PDI <0.1) and scalable Production. (3) Active ligands (e.g., cRGD peptides boosting tumor uptake 4.2×) and oxygen-independent radicals (e.g., AIPH generators) to overcome hypoxia resistance, enhanced targeting and efficacy. (4) Low-power pulsed lasers (<0.3 W/cm²) reducing thermal damage and standardized biocompatibility frameworks (ISO/ASTM) for regulatory approval.

Despite the immense potential of NIR-II materials for deep tissue penetration, targeted therapy, and precise drug release, there remain several challenges in terms of technological implementation. From optimizing NIR-II light sources to enhancing the biocompatibility and targeting capabilities of materials, as well as developing multimodal systems, further in-depth research and innovation are required. The specifics are as follows:

1. Optimization of NIR-II Light Sources and Imaging Technology: Although NIR-II light sources exhibit excellent tissue penetration, current NIR-II light sources still face some limitations. For example, NIR-II laser sources require high power, which may lead to thermal damage or discomfort in the surrounding tissue. Optimizing the performance of NIR-II light sources to ensure deep tissue penetration while minimizing unnecessary side effects remains a critical issue that requires resolution.

2. Hypoxia Micro-targeting and Precise Drug Release Designing: nanocarrier materials capable of targeting hypoxic regions and precisely releasing drugs under NIR-II light remains a technical challenge. The balance between biocompatibility, stability, and targeting ability of the materials directly impacts the effectiveness of drug delivery. Moreover, the diffusion and controlled release efficiency of the drug within the tumor microenvironment also require further improvement.

3. Development of NIR-II Light-Responsive Materials: The development of NIR-II light-responsive nanomaterials faces numerous challenges. These materials must exhibit strong light absorption or fluorescence characteristics in the NIR-II wavelength range while avoiding clearance by the immune system and addressing concerns related to biodegradability and toxicity.

4. Complexity of Multimodal Systems: Multimodal systems integrate imaging, drug delivery, and therapeutic functions. However, ensuring seamless coordination between different modalities to ensure precise synergy between imaging and therapeutic functions remains a significant challenge. Particularly in tumor treatment, the interactions, efficiency, and optimization between different functional modules still require extensive research and experimental validation.

With continuous advancements in material science, nanotechnology, optical imaging technology, and biomedicine, NIR-II light-responsive materials hold great promise in tumor therapy. In the future, they are expected to become an essential tool in clinical cancer treatment.

7. Conclusion

NIR-II materials demonstrate transformative potential in hypoxic tumor theranostics through superior deep-tissue imaging and oxygen-independent therapeutic mechanisms, yet face critical challenges including toxicity risks from long-term organ retention, scalability limitations due to rare-element dependency and high production costs, and suboptimal tumor targeting. Addressing these requires interdisciplinary strategies: developing renal-clearable ultrasmall probes, enzyme-degradable polymers for enhanced biocompatibility, standardized clinical validation, and integrated “all-in-one” platforms combining deep-penetration imaging with hypoxia-modulating therapy to accelerate clinical translation.

Funding Statement

This work was partially supported by Shanghai Science and Technology program [23010502600], “Shuguang Program’’ supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission [23SG58], Special Project on Science Popularization under the Science and Technology Innovation Action Plan [24DZ2302800], Shanghai University of Medicine and Health Science Seed Fund [SSF-24–21–01], Construction project of Shanghai Key Laboratory of Molecular Imaging [18DZ2260400].

Article highlights

• NIR-II Optical Superiority: NIR-II materials (1000 – 1700 nm) enable deep-tissue penetration (>5 mm), reduced scattering, and minimized autofluorescence, surpassing NIR-I limitations for hypoxic tumor imaging.

• Hypoxia-Sensing Probes: Activatable probes (e.g., TBTO) exhibit > 4.1× fluorescence enhancement under hypoxia, allowing real-time monitoring of tumor oxygen gradients.

• Multimodal Imaging Integration: NIR-II fluorescence/photoacoustic imaging combined with MRI/CT provides structural-functional insights into hypoxic TME, achieving >2 cm penetration depth (e.g., BLICP@O₂ platform).

• Oxygen-Independent Therapies: Radical generators (e.g., AIPH) and Type-I PDT agents (e.g., Y6-Th) overcome hypoxia constraints by producing cytotoxic species (•R, O₂•⁻) without oxygen dependency.

• Catalytic Oxygenation: MnO₂-based nanosystems decompose endogenous H₂O₂ to O₂, increasing tumor oxygenation by 43% and enhancing PDT efficacy.

• Synergistic Nanoplatforms: “All-in-one” designs (e.g., TAT-Pd@Au/Ce6/PAH/H-MnO₂) integrate nucleus-targeted PTT (56.9% efficiency) with hypoxia-relieved PDT for complete tumor ablation.

• Clinical Translation Roadmap: Low-power lasers (<0.3 W/cm²) reduce thermal damage, while ISO/ASTM biocompatibility frameworks accelerate FDA approval.

• Future Direction: Enzyme-degradable polymers (e.g., MMP-responsive SPNs) and cRGD-targeted ultrasmall probes represent next-generation theranostics for precision hypoxia modulation.

Author contributions

Qianqian Cai: Conceptualization, Writing – Original Draft.

Haoyu Zhang: Data Curation, Visualization.

Zhaohang Huang: Formal Analysis, Validation.

Qiwei Tian: Supervision, Funding Acquisition

Disclosure statement

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.

Reviewer disclosure

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/17435889.2025.2525057