Application of nanomaterials in early diagnosis of cancer

2.1 Noble metal NMs

NM-NMs refer to noble metals with a size below 100 nm, including platinum (Pt), silver (Ag), gold (Au), and so on. NM-NMs have gained significant attention because of their distinct properties. First, the reinforcement of the light field leads to broad scattering and strong light absorption. The strength of the photothermal performance of NM-NMs is determined by the resonance of the free electrons under light, also referred to as local surface plasmon resonance (LSPR). LSPR has considerable applications in optics and imaging due to its ability to radiate light [20]. Second, the enhancement of the magnetic field leads to the resonance of the nucleus. The spin nucleus absorbs the electromagnetic wave at a specific frequency, and the spin energy from the lower level to the higher level generates a highly sensitive signal.

NM-NMs are still dominated by Au NMs. Au NPs with different shapes have been fabricated, such as rod-like [21–23], shell-like [24–26], and flower-like [27] NPs. Au NPs with different shapes exhibit distinct advantages, which can provide higher spatial and temporal resolution for diagnostic imaging in the early detection of cancer. Au NPs possess desirable optical physical properties that can enhance the signals of Raman [28,29] and fluorescence imaging (FI) [30,31]. A new bifunctional imaging nanoprobe containing Au NPs and nanocluster (NC) composite material [32] has been proposed. Au NPs are used to enhance fluorescence and Raman signals, while Au NCs can be used as a fluorophore in the imaging of metal-enhanced fluorescence (MEF). Results have shown that this novel dual-functional imaging nanoprobe has superior biocompatibility and excellent surface enhancement Raman scattering (SERS) and MEF imaging efficiency, which can achieve an accurate diagnosis at early stage of cancer. With the regulation of external environmental factors, Au NPs can also improve the contrast of computerized tomography (CT) imaging [33,34], enhance magnetic resonance imaging (MRI) signals [35], and produce strong photoacoustic signals [36].

Au NCs are aggregates composed of several or dozens of Au atoms. The number of Au atoms in aggregates is a major factor affecting luminescence. Feng et al. [37] synthesized ultra-small fluorescent Au NCs. After FI, red fluorescence can be clearly visualized in the cytoplasmic region of the breast cancer cell line 4T1, and the red fluorescence intensity increases with an increase in incubation time (Figure 2a). In contrast, almost no red fluorescence was detected in 293T normal cells (Figure 2b). New Au NCs coated with methionine have also been prepared. After incubation with the Au NCs for 1 h, A549, Hela, MCF-7, and HepG2 cells exhibited yellow fluorescence, whereas the normal cells showed none [38]. To make a preliminary evaluation of the biocompatibility of Au NCs, MTT assays were performed using 4T1 (mouse breast cancer) and 293T (human embryonic kidney) cells as models. The MTT results show that Au NCs had excellent biocompatibility because cells can survive even up to a relatively high concentration (Figure 2c). Therefore, we can conclude that the prepared Au NCs are non-cytotoxic and can be used for cell imaging applications.

Figure 2

Confocal laser scanning microscopy imaging of 4T1 cancer cell (a) and 293T normal cell (b) incubated with AuNCs, respectively, at 37℃ with different times (2, 4, and 8 h); cell nucleus was stained blue by Hoechst 33258. (c) Cell viability of 4T1 cancer cell (a) and 293T normal cells (b). After 24 h of co-incubation with different concentrations of Au NCs by the MTT assay. Reproduced with permission from the study of Feng et al. [37]. Copyright 2020 Elsevier.

NPs of Ag and Pt have also been studied in detail for their distinct potential for light absorption in the biological field and significant surface plasmon resonance properties to prepare imaging probes, which have substantially contributed to the early diagnosis of cancer. Recent research results have shown that a novel nanoplatform (carbon nanohorns/Pt NPs/DNA) using clamp hybridization chain reaction (c-HCR) can be used for imaging of the intracellular Zn²⁺ [39]. Carbon nanohorns/Pt NPs/DNA nanoplatforms incubated MCF-7 cells have shown a red signal more obviously. The nanoprobe without DNA-zyme functionalization incubated with MCF-7 cells displayed a weak red signal, indicating the sensitivity of the novel nanoplatform to Zn²⁺ Au–Ag alloy NPs has also been synthesized, and they can combine with dual-energy mammography for early detection of breast cancer. Au–Ag alloy NPs possess more effective dual-energy breast X-ray contrast performance and improved biocompatibility [40], which can enhance the sensitivity of early cancer detection. In addition, cell viability results indicate that the inclusion of 30% or more gold in the formulation renders the Au–Ag alloy NPs biocompatible (Figure 3a). Then, according to the biodistribution of Au–Ag alloy NPs, researchers found that the NPs have a long circulation time and significant amounts of NPs accumulated in the tumor because of passive targeting delivery. In addition, the NPs are found in the bladders and intestines of the mice (Figure 3b), which suggests that there is some excretion of the NPs via urine and feces.

Figure 3

(a) Cell viability of J774A.1 and Hep G2 cells when incubated with Au–Ag alloy NPs. (b) In vivo CT imaging with Au–Ag alloy NPs. 3D volume rendered CT images of a mouse (without a tumor) injected with GSAN. Green arrowheads indicate blood vessels, and green circles indicate the bladder. Hearts, kidneys, intestines, and bladders are labeled H, K, I, and B, respectively.

2.2 Carbon-based NMs

Carbon-based NMs are carbon materials with a well-dispersed phase whose size is less than 100 nm. Data indicate that carbon-based NMs coupled with various biomarkers can provide good resolution in the early diagnosis of cancer. The localization of target cells has also been confirmed [41].

As new zero-dimensional carbon NMs, carbon dots (CDs) have excellent optical properties. They are commonly used in early diagnosis of cancer because of their features to penetrate cancer cells. Compared with organic dyes, their narrow emission bands can produce higher-resolution FI, allowing the accurate location of cancer cells [42]. A ratiometric fluorescent sensor was synthesized based on blue fluorescence CDs. The sensor showed specificity in the fluorescence sensing of Cathepsin B activity with a trace detection limit (<0.01 ng/mL). Rashidi et al. [43] synthesized CDs with pH-dependent behavior, which can enter cancer cells more comprehensively through receptor-mediated endocytosis and distinguish between cancer cells and normal cells by FI. Fluorescent CDs with magnetic mesoporous silica NPs were conjugated to construct a biocompatible nanoplatform [44]. The fluorescence signals remained bright and stable after incubation for 24 h with HeLa cells, improving the detection rate for early diagnosis of cancer.

One-dimensional carbon NMs, due to their excellent electrical, magnetic, and optical characterization, have attracted attention increasingly [45,46]. Carbon nanotubes (NTs) are a typical representative of one-dimensional carbon NMs. Xiang et al. studied antibody-functionalized single-walled carbon NTs, which showed excellent biocompatibility. Due to the feature including strong light absorption in the near-infrared (NIR) region, high transparency in biological tissues, and low absorbance [47], NTs can be used in the early detection of cancer by photoacoustic molecular imaging in vivo. The in vivo and the in vitro results have shown that carbon NTs with anti-integrin α_vβ₃ molecules exhibit high contrast and active targeting in the detection of human glioblastoma tumor, indicating that photoacoustic molecular imaging of carbon NTs may be a more effective method in early diagnosis of cancer.

Graphene, a representative of two-dimensional carbon NMs, has recently drawn attention in early diagnosis and imaging of cancers. In recent years, graphene oxide (GO) has gained significant attention in the field of early cancer diagnosis imaging due to its biocompatibility, unique physicochemical properties, and stability in biological media [48]. A study [49] has confirmed that NMs of multi-targeted GO can detect dissimilar types of mRNA in living cells simultaneously. This nano-sensing platform can detect endogenous mRNA of β-actin and Mn-SOD mRNA in cancer cells, showing a high specificity, rapid response, good biocompatibility, and nuclease stability. Fan et al. [50] used the reduced GO nanosheet-assisted fluorescent probes to construct functional nanosystems for in situ and real-time monitoring of key genes (p53 and p21 mRNA) in liver cancer. As shown in Figure 4a, both red and green fluorescence in HepG2 cells correspondingly reflected p53 and p21 mRNA levels, which increased in a time- and concentration-dependent manner. Compared with traditional gene detection methods, the new technique exhibited increased sensitivity and outstanding selectivity. The functional nanosystem was applied to monitor cisplatin-induced dynamic changes in mRNA in HepG2 cells (Figure 4b), which approach could be used to reveal the regulation of p53 and p21 mRNA in cancer cells (Figure 4c).

Figure 4

(a) As the treatment time increased from 0 to 48 h, the fluorescence signals corresponding to P53 and P21 in HepG2 cells were gradually increased under a laser confocal microscope. (b) The expression of p53 and p21 mRNA in HepG2 cells increased gradually after treatment with cisplatin at 0–8 μM. (c) Detection of p53 and p21 mRNA in HepG2 cells treated with cisplatin at different times and concentrations. Reproduced with permission from the study of Fan et al. [50]. Copyright 2019 Elsevier.

Besides its unique G-band of Raman spectra, GO has more excellent biocompatibility and solubility when compared with carbon NTs and can be used as a Raman probe to image cells or tissues sensitively [51]. Utilization of the surface enhancement effect, Qing et al. used Au NPs to decorate GO and obtained Raman probes of Au/GO hybrids. Hela 229 cells were accurately imaged after endocytosis. This study also suggests that GO has great potential as a detection tool for tumor biological systems.

2.3 Magnetic NPs

Magnetic NPs are a major class of NMs, which significantly affects the results of disease diagnosis. The diagnosis could be manipulated by remote magnetic fields, thus being beneficial for the in vivo applications [52]. Iron oxide nanoparticles (IONPs) have become a new alternative to conventional contrast agents in MRI because of the unpaired electrons outside the iron core. The high-speed rotation of extranuclear electrons produces a net magnetization vector, so IONPs have strong paramagnetism. And the IONPs have high biocompatibility, providing an alternative to traditional contrast media in MRI [53]. Gulzar et al. [54] synthesized a nanorod containing α-Fe₂O₃, which was used as a T2-MRI contrast agent after being coated with polyethylene glycol (PEG). The results show that the nanorods display high cell viability of 85–98% in an overall concentration range of 500–15.6–500 μg/mL after an incubation period of 24 h, hence suggesting that the nanosystem displays small cellular toxicity. The nanorods produced excellent contrast of T2 MRI. Further, research on T2-weighted MRI was performed on tumor-bearing mice. The mice injected with NPs showed a more apparent attenuation effect of T2 signals within the image than the control animals (Figure 5a), indicating the fact that Fe₂O₃ nanorods have extraordinary superiority in early-cancer imaging. Based on this, Fe₂O₃@Au core–shell NPs have also been prepared as a contrast agent for T2 measurement in MRI [55]. At 30 min after a subcutaneous injection of Fe₂O₃@Au NPs into the tumor-bearing mice, the Hounsfield unit value measured by CT increased from 40 to 700. In MRI, the tumor area before and 30 min after injection was increased from 1015 to 1904, and the tumor area was significantly darkened. The NPs enabled MRI/CT dual-modality imaging of tumor-bearing nude mice. Further, Fe₂O₃@Au NPs were expected to be a targeted probe in the early-cancer imaging.

Figure 5

(a) NIR thermal images of mice treated with PBS and α-Fe₂O₃-PEG. Reproduced with permission from the study of Gulzar et al. [54]. Copyright 2022 Springer. (b) Schematic illustration of the conjugation between antibodies (anti-alpha-fetoprotein [AFP] and anti-GPC3) and ultrasmall SIO NPs. Reproduced with permission from the study of Ma et al. [62]. Copyright © The Authors 2019. (c) In vivo fluorescence images of tumor accumulation and tissue distribution for SIO nanoprobe in MGC803 human gastric tumor-bearing athymic nude mice. Reproduced with permission from the study of Wang et al. [63]. Copyright 2011 BioMed Central Ltd.

Superparamagnetic iron oxide (SIO) as an MRI contrast-medium has been extensively studied. SIO NPs, with a diameter of 10–100 nm, have a crystal core of Fe₃O₄ or γ-Fe₂O₃. It can generate intense magnetism in a weaker magnetic space while the induced magnetism disappears because of the removal of the external magnetic space. Superparamagnetic agents can significantly reduce the T2/T2* relaxation time [56]. SIO with good superparamagnetism affects the proton spin relaxation behavior of water molecules around the affected tissue, improves the accuracy of T2 imaging [57], and helps detect cancer cells within normal tissues. Therefore, SIO NPs are regarded as an ideal MRI negative contrast agent [58–61]. Ma et al. [62] developed a dual-antigen targeted MRI probe using ultrasmall SIO NPs (5 nm) modified with alpha-fetoprotein and glypican-3 antibodies (Figure 5b). The targeting efficiency and MRI sensitivity of this bispecific probe to hepatocellular carcinoma cells were extremely high, which provided a basis for clinical application in early cancer detection. Wang et al. [63] developed a multifunctional SIO nanoprobe targeted gastric cancer with the capability of MRI and FI, simultaneously. The SIO nanoprobe was conjugated with BRCAA1 monoclonal antibody, owning active targeting features. The nanoprobe was injected into a model mouse of stomach carcinoma via the tail vein, and imaging results indicated that the NPs could target carcinoma of stomach cells after the combined imaging. As shown in Figure 5c, fluorescence signals are produced within 30 min after injection, and the maximum contrast occurred at 6 h post-injection, providing a research idea for dual-model imaging of early gastric cancer.

2.4 Rare-earth NMs

Rare-earth elements have special (superior) optical, electrical, and magnetic properties, which are attributed to their special electronic configuration. Rare-earth NMs have the advantage of rare-earth components and NMs, which have been applied in tumor diagnosis, disease treatment, and biological imaging [64]. Among various bioimaging methods, FI technology based on rare-earth-doped NPs can visually display the cell viability of living animals [65] and is a powerful tool in medical and biological fields.

Gadolinium (Ga), a silvery white rare-earth metal, exhibits ferromagnetism and remarkable superconductivity. It is widely used in MRI contrast agents for clinical diagnosis of disease. Ga-chelated perfluoropentane-polylactic-glycolic acid copolymer NPs have been synthesized [66] to specifically detect thyroid carcinoma. Jiang et al. [67] first synthesized Ga-doped silica NPs with a reverse microemulsion process, connecting YPSMA-1 to its surface (Figure 6a). The NPs had good stability and a better longitudinal relaxation rate. The in vitro cellular targeting test proved that Ga-doped NPs could recognize the specific antigen of prostate cancer cells and emit green fluorescence (Figure 6b). Thus, it can be used as a contrast agent for specific MRI early diagnoses of prostate cancer cells.

Figure 6

(a) Illustration of Gd@SiO₂-Ab synthesis strategy. (b) CLSM images of LNCaP cells incubated with Ga-doped NPs for 2 h. Reproduced with permission from the study of Jiang et al. [67]. Copyright 2018 Elsevier. (c) MRI and PAI performance of CaF2:Y,Gd,Nd NPs. Reproduced with permission from the study of Yu et al. [70]. Copyright © 2022 by the authors.

Rare-earth doped upconversion NPs are a new type of luminescent material with unique luminescent properties. The material can emit UV or visible-upconversion fluorescence under NIR excitation and can be widely used in biological monitoring such as CT and MRI. Huang et al. [68] synthesized NPs (folic acid [FA]–PEI–NaGdF4:Eu) as new magnetic fluorescent nanoprobes for cellular imaging with MRI/phosphorescence dual-modality. Besides its suitability for FI, the probe has a high longitudinal relaxation rate suitable for T1-weighted MRI. With the modification of FA, the dual-modality nanoprobe could identify carcinoma with a plenty of folic acid receptor. Wang et al. [69] synthesized rare-earth upconversion NPs with a paramagnetic lanthanide composite shell. The cytotoxicity tests were conducted for rare-earth upconversion NPs on normal L929 cells. The results show good biocompatibility of NPs. The viabilities of L929 cells are over 85% when incubated with NPs even at 400 µg/mL for 48 h. Such good cell biocompatibility is mainly attributed to the PEGylation of NPs. The in vitro imaging of cancer cells achieved the satisfactory imaging contrast of rare-earth upconversion NPs. The in vivo FI and T1-weighted MRI showed that the fluorescence and MRI signals in mouse liver were markedly enhanced after the NPs were given. All results have shown that the high upconversion fluorescence and strong relaxation rare-earth NPs with dual-modality of FI and MRI provided great applications foreground.

Meanwhile, rare-earth NMs, such as ytterbium (Yb), neodymium (Nd), and erbium (Er), have been gradually developed for bioimaging in early diagnosis of cancer. Yu et al. [70] doped rare-earth ions Yb, Ga, and Nd NPs to calcium fluoride to design a multimodal imaging probe with high crystallinity, uniformity, and biosafety. The in vitro and in vitro experiments showed the efficient multimodal imaging capabilities of the nanoprobe in NIR-II, fluorescence, photoacoustic, and MRI (Figure 6c). In another study [71], PEGylated NPs doped with Yb, Nd, and Er were prepared to concurrently image NIR-II and NIR-IIb windows. With the complementarity of rare-earth NPs and PEG, NPs have higher temporal and spatial resolutions and hydrophilic surfaces, providing a more precise and accurate diagnostic image. Thu Huong et al. [72] prepared a novel nanocomposite doped with Yb and Er to label cancer cells. The nanocomposite exhibited excellent physiological medium dispersibility and high upconversion luminescence through nonlinear optical processes. The in vitro experiments further showed that the positively charged nanocomposite endowed its surface with a dangling bond of Yb3+/Er3+, which could specifically bind to the MCF-7 breast cancer cell membrane. Under the excitation of a specific wavelength, the fluorescence from the surface of cancer cells could be detected. Under similar experimental conditions, healthy cells could not be detected by the nanocomposite. In general, these NPs containing rare-earth components were expected to become ideal biomedical markers and early diagnostic benchmarks for cancer.

In addition, self-propelled micro/nanomotors have emerged as a promising platform in recent years. Most of the reported micro/nanorobots are composed of inorganic NMs [73], such as noble metal NMs and rare-earth NMs, which can effectively convert various energy sources into power and realize autonomous motion. They have gained considerable attention in the field of early cancer diagnosis. These micro/nanomotors not only inherit the excellent properties of micro/NMs but also possess good biocompatibility and biodegradability. These intriguing advantages of the micro/nanomotors hold significant potential for precise cancer treatment [74].

3.1 Metal-organic framework (MOF)

In recent years, NMs represented by organic framework materials have been widely used in FI, sensing, and other fields. Organic framework materials have been becoming ideal materials for the detection of cancer markers because of their unique structure.

MOFs contain the structure of intramolecular pores which are formed after the self-assembly of metal ions or their clusters with organic ligands through coordinate bonds. MOFs have inherent advantages, such as structural diversity, large surface area, and excellent electrical conductivity, which are attributed to the properties of metal nodes and organic linkers [75]. Recently, MOFs have been applied in the detection of early-stage cancer biomarkers [76–78]. Several studies have shown that bi-MOFs (containing two metals) have high sensitivity in early diagnosis of cancer. They can be used as aptamer sensors to detect various indicators, particularly for markers of early cancer. Compared with single-parent MOFs, bi-MOFs usually show enhanced skeleton fluid stability. Zhou et al. [79] designed a new bi-MOF to detect tyrosine kinase-7 (PTK7) protein. The sensor (Zn-MOF-Zr-MOF) had ultralow detection limits. Wang et al. [80] designed a type of bi-MOF containing the elements Tb and Fe (Figure 7). The bi-MOF had porous nanostructures as well as possessed the physical and chemical properties of bulk materials, which significantly improved the sensing performance. The unique luminescent properties were also conducive to further cell imaging applications. Combined with electrochemical impedance spectroscopy (EIS), Tb-MOF-on-Fe-MOF bi-MOFs were used to detect CA125 and MCF-7 tumor cells in serum samples, and the detection performance was satisfactory. The RSD ranges were 1.50–4.64% (CA125 cells) and 2.48–3.76% (MCF-7 cells), respectively. Thus, bi-MOFs provided higher accuracy in practical applications and presented considerable advantages in early diagnosis of cancer.

Figure 7

Diagram of a bi-MOF structure containing Tb and Fe ions.

Covalent organic framework (COF) materials are a highly orderly arranged multi-hole crystalline organic polymer [81]. Compared with ordinary MOFs, COF materials have significantly improved stability due to their stronger coordination bonds. COF materials are also characterized by large specific surface area and lower density. They have received extensive attention in optoelectronics [82], sensing [83], catalysis [84], and biomedicine [85,86]. Yan et al. [87] synthesized a COF of a porphyrin-based (p-COF) sensor via a simple oil bath method. The p-COF sensor had a nanosheet structure, a large cavity, and rich nitrogen-containing groups. The microscopic structure of the groups enabled the aptamer chain to be firmly fixed. The p-COF was used to immobilize the aptamer chain targeting receptor of epidermal growth factor (EGFR) to identify EGFR and live MCF-7 cells. The interaction between EGFR and the aptamer chain changed the signal of electrochemical. The electrode could thus be effectively detected the substrate with the ideal linear range of 0.05–100 pg/mL. The p-COF material could detect living MCF-7 cells, indicating the potential of p-COF in the detection of aimed cancer cells or their markers. Li et al. [88] prepared a porous nano-COF by polycondensation. The framework consisted of hexaketone cyclohexane and octahydrate hexaketone cyclohexane. It was characterized by higher specific surface area, significant electrochemical performance, and good biological affinity for the aptamer chain. The framework was a bifunctional aptasensor detecting K7M2 cells and vascular endothelial growth factor (VEGF) 165. The results showed that the LOD of VEGF165 was 0.18 fg/mL. The LOD of VEGF165-overexpressed osteosarcoma cells (K7M2 cells) was 49 cells/mL. The sensor was further used to detect human serum samples with an average apparent recovery rate of 97.41%, paving the way for the practical application of COF NMs.

3.2 Quantum materials

The size of quantum materials generally ranges from 1 to 100 nm. Owing to the quantum confinement effect, QDs possess superior properties, such as narrow excitation and emission bands and a tunable emission wavelength [89]. In recent years, quantum materials have drawn extensive attention in biomedicine, including tissue imaging (bioimaging), diagnosis, single-molecule probes, and drug delivery [90].

CDs own unique advantages, such as lower cost, chemical inertness, ease of functionalization, and biocompatibility. Omer et al. [91] used CDs to examine the optical ultra-sensitivity of biosensors of antigen 125 (CA-125) in the early stages of ovarian cancer. CA-125 detection substantially depended on the fluorescence intensity of CD-embedded polymers, which was significantly quenched after interaction with CA-125 under optimized conditions. The results showed that the LOD was 0.66 U/mL. There was a good linear relationship in the range of 0.01–129 U/mL (R ² = 0.99). The method was more simple, sensitive, time-saving, and cost-effective than traditional CA-125 quantitative methods in clinical trials. Li et al. [92] prepared hydroxyl-modified CDs for the detection of desmin in the serum from patients suffering rectal cancer. The CDs exhibited better specificity and avoided the interference of immunoglobulin G, AFP, and carcinoembryonic antigen (CEA). The LOD of the probe was less than 1 ng/mL, fully meeting the requirements of desmin detection in human serum. This study proposes a promising strategy for assessing colorectal cancer with lower cost and more excellent sensing performance.

Graphene quantum dots (GQDs) provide a new possibility for the development of nanosensors because of their special features, such as quantum confinement, edge effects, and abundant chemical modification sites. Kalkal et al. [93] prepared a fluorescent biosensor by utilizing GQDs as energy donors. The biosensor could detect the biomarkers of small-cell lung cancer ultrasensitively. The biosensor had an advantage of LOD, a wider detection coverage, and a faster response time. The fluorescent sensor performed efficiently in the sample tests of human blood, in which the average recovery rate was up to 94.69%. Ganganboina and Doong [94] prepared an immunosensor of GQDs for the quantitative detection of CEA. The sensor was a probe to identify antigens of CEA and increase electrochemical activity. The coupling of CEA antibody modified on the surface of GQDs with CEA antigen in the blood statistically enhanced the resistance of charge transfer, and the LOD was up to 0.01 ng/mL. Moreover, the immunosensor was applied to detect the CEA antigen in the sample solutions with a higher recovery rate from 96.0 ± 2.6 to 103 ± 3.8%.

CdSe quantum dots (CSQDs) have the advantages of high fluorescence quantum yield and easy detection, which has been widely used in the marker detection of early cancer. In a previous study [95], a nanobiosensor containing CdSe@CdS quantum dots was prepared. The LOD of the biosensor for miR-106a and let-7a were 0.06 fM and 0.02 fM by differential pulse voltammetry (DPV), respectively. The results not only confirmed the successful construction of the expected biosensor platform but also showed its acceptable stability, appropriate selectivity, significant sensitivity, and strong specificity of the platform. On the basis of the fluorescence characteristics of CSQDs, Liu et al. [96] designed a new pH detection method using CSQDs as sensitive substances; the quantum dots were found to exert better effects than other pH detection materials, with a measurement resolution of 0.037 pH in the pH 6.1–7.8 range. The quantum dots also showed high pH detection potential to achieve efficient and accurate pH measurement in the acidified environment of cancer cells with abnormal metabolism behavior.

In addition, with the rapid development of quantum dots in the early diagnosis of cancer in recent years, some novel quantum dot materials have also drawn interest from the scientific community. Ganesan et al. [97] prepared the aluminum-based quantum (AlQ) with a 3D biocompatible structure. The prepared AlQs were able to detect the femtomolar concentration of substrate (10–15 M). Researchers cultured pancreatic carcinoma cells (ASPC-1) and carcinoma of lung cells (H69) with AlQs to prove the potential application of AlQs. The results showed that the quantum probes were accurately internalized by cancerous cells without harm to the normal cells. The results suggested that aluminum-based NPs may be used in the in vitro cancer detection. A 3D quantum probe based on ZnO was also fabricated to diagnose cancer in vitro [98]. The probe size was reduced to the quantum scale, and reinforcement of SERS could be observed. In addition, the probes fixed on 3D nanodendrite platforms promoted cell adhesion and proliferation. The quantum probes efficiently distinguished cancer cells from normal cells and sensed biomolecules such as DNA, RNA, proteins, and lipids in vitro.

3.3 Aptamer NMs

Aptamers are short man-made oligonucleotides composed of RNA or DNA. The aptamers have high specificity to target molecules. Aptamers are considered promising NMs for early detection of cancer. The binding principle of aptamers with target molecules is similar to that of the antigen–antibody interaction. However, aptamers have several advantages, such as easy-to-synthesis, stability, inexpensiveness, and more sensitivity due to their smaller size, that are significantly better than antibodies. After decades of research, aptamer technology has been widely used in molecular recognition, cancer diagnosis, and drug research [99,100,101].

DNA aptamers have higher specificity to detect the biomarkers of different cancer cells. Xiao et al. [102] developed a nanofiber of DNA aptamers for the effective identification of circulation tumor cells (CTCs). The DNA aptamers were coupled onto the surface of the nanofibers via a thiol-maleimide reaction. The formed aptamer nanofibers could specifically capture CTCs with 87% capture efficiency. After the incubation of nuclease, aptamer-nanofibers release 91% of the captured cancer cells without damage to these cells. These NMs have great prospects in the capture and release of CTCs, which can be used in the cell-sorting field. Compared to the binding efficiency of a single aptamer nanoprobe to CTCs, that of a dual-aptamer nanoprobe to CTCs was significantly improved. Zhang et al. [103] conjugated the aptamers SYL3 and CSgc8 onto the surface of AuNPs to fabricate an ultraefficient dual-aptamer single-CTC sensor. The sensor could find 93.6% of single CTC in the blood with a measurement efficiency of 73.8%.

To enhance the stability and operability of DNA NMs, Huang et al. [104] developed a thermally stable DNA nano-sieve. The results showed that the value of melting temperatures of the single-chain DNA exceeded 80°C. The genetic materials could be stably fixed on the sensor at a capture temperature of 65°. After ambient temperature storage for 6 months, the DNA nano-sieve still had a capture capacity for low-concentration samples. The device was further used to specifically capture cy5-labeled ssDNA samples. A significant change in blue fluorescence on the device was observed with the naked eye, whereas the nontarget sequence-modified nano-sieve caused no color change on the device. Moreover, the device efficiently detected the desired DNA with one-eighth of concentration (1 μt), which can be used for ctDNA enrichment in the future to facilitate the diagnosis and clinical analysis of early cancer.

In traditional aptasensors, the electrode surface is directly assembled with aptamers. However, the entanglement, aggregation, and steric hindrance effects of traditional aptamers on the sensor surface decrease recognition efficiency. This problem has been addressed by fixing the DNA tetrahedron on the Au electrode, establishing a DNA nanostructured aptamer sensor. The sensor can capture human hepatocellular carcinoma cells (HepG2) specifically and effectively [105]. The cell sensor had a wide detection range from 10² to 10⁷ cells/mL. Further exploring the applicability of this aptamer sensor, HepG2 cells were added to human blood in vitro to analyze the recovery results. The recovered cells were consistent with the number of the added HepG2 cells, indicating that the cell sensor could be a useful tool for diagnosis of cancer early. Jiang et al. [106] also prepared a DNA nanotetrahedral aptasensor. This aptasensor used DNA as a recognition and capture unit and further coupled with Au NPs-DNA to achieve signal amplification. The LOD was shown to be 1.66 × 10⁴ particles/mL. This sensitive and multifunctional electrochemical aptasensor provides an approach to analyze the exosome protein spectrum and can be potentially used in early cancer diagnosis.

3.4 Carbon NMs

Carbon is a common natural element. By changing the hybrid status of sp, sp2, and sp3, varieties of carbon allotropes can be synthesized. Researchers have gradually found that NMs based on carbon allotropes have a large surface area, good biocompatibility, and conductivity. Carbon can be used to quickly, easily, and sensitively detect various early cancer markers owing to their advantages.

First, carbon NMs can be combined with electrolytes and biological probes to accelerate signal transmission, enhance analytical sensitivity, and decrease LOD [107,108]. Second, carbon NMs can be used as electrode support substrates or media to regulate electron transfer [109]. Owing to these advantages, carbon NMs could significantly improve the performance of electrochemical sensors. Ji et al. [110] designed a biosensor to detect the biomarkers of prostate cancer by using biologically activated multi-walled carbon NTs and microporous filter paper. Multi-walled carbon NTs were modified with an anti-prostate specific antigen monoclonal antibody, and the activated carbon NTs were deposited on a microporous filter paper. The changes of resistance were induced by the site-selective interplay between PSA-antigen and PSA. The prepared sensor could detect PSA of 0–500 ng/mL within 2 h by measuring the changes of resistance, with the LOD of 1.18 ng/mL. The prepared sensor exhibited satisfactory detection range and sensitivity. It was paid for about 5% of the cost of an enzyme-linked immunosorbent assay, but ten times faster than that of the enzyme-linked immunosorbent assay. These benefits could improve the diagnosis of prostate cancer (PSA > 4 ng/mL).

Carbon NMs have been combined with other NMs to form nanocomposites, further improving the detection performance of the sensor. The resulted nanocomposites have been developed for the construction of biosensors. Zhang et al. [111] developed an electrochemical cytosensor for detecting cell lines of non-small cell lung cancer. Monodisperse colloidal carbon nanospheres were synthesized using a microwave-hydrothermal method. Au NPs were placed on the surface of the carbon NPs in a self-assembly process to obtain a 3D structure microsphere. The voltammetric signal and detection sensitivity were significantly improved due to the synergistic effect of carbon nanospheres and Au NPs. Antibodies against the CEAs were then immobilized on the microspheres to obtain a cytosensor. The sensor could detect the concentration range from 4.2 × 10⁻¹ to 4.2 × 10⁻⁶ cell/mL. In another study [112], a dual-mode biosensor was designed. The Au electrode of the biosensor was first coated with GO, and ssDNA was then connected to the functional group of GO, which could simultaneously detect VEGF and PSA in human serum. The results showed that the LOD of the biosensor for VEGF was 0.05 ng/mL and that for PSA was 1 ng/mL, which can be used for early diagnosis of prostatic carcinoma. Dutta et al. [113] developed a new sensor with GQDs and carbon nanohorns to detect the cancer biomarker. The immunosensor had the LOD of 0.25 pg/mL. The recovery rate of biomarkers from human serum exceeded 99.0%.

3.5 Noble metal NMs

Noble metal NPs are important not only in early detection of the in vivo diagnosis for cancer but that of the in vitro diagnosis. Sun et al. [114] developed a silver-amplified fluorescent immunoassay method for the detection of disease biomarkers by combining a silver NP-associated immunoassay with a fluorescent Ag⁺ probe based on aggregation-induced emission. The results showed that the LOD of the probe was 42 pg/mL. This method could detect AFT in serum samples of patients with hepatocellular carcinoma. The findings proved the potential of silver-amplified fluorescent immunoassay in clinical diagnosis. A sandwich immunoassay based on SERS has also been performed for the ultrasensitive test of AFT [115]. This sensor was composed of Exf-MoS₂ fixed with mAb and BSA, which mAb was responsible for quickly capturing the α-FTP, adding Ag@AU NPs onto the Exf-MoS₂ surface to enhance the signal (Figure 8). The immunosensor had good reproducibility, which the LOD was 0.03 pg/mL. He et al. [116] designed a sensor based on the magnetic beads with bimetallic NP and dual aptamer. Metal NPs could produce inductive coupling plasma mass spectrometric signals, and the sensor was labeled to identify cancer cells. The results indicated that the technique could be used to identify at least 50 cancer markers, and the RSD was less than 5%.

Figure 8

Silicon wafer of the sandwich immune complex with Ag-coated Au nanocubes.

4.1 Bioimaging

Bioimaging can be divided into optical and magnetic imaging. Optical imaging is based on distinct absorption spectra of different materials. The spectra generate image contrast of specific molecules, which is used for imaging. Magnetic imaging is used to enhance image contrast, regulate signal intensity by changing the regional magnetic field conditions, or change the tissue relaxation time (T1/T2) in the magnetic field using NPs. Bioimaging methods for early diagnosis of cancer mainly include Raman imaging [117], FI [118,119], NIF imaging [120], MRI [121], and so on. NMs are the material basis of biological imaging, and their features are crucial to the detection results. The strong signal generated by NM imaging helps overcome the low sensitivity of MRI and the limited penetration depth of optical imaging [122]. The diagram for the mechanisms of NM in early diagnosis of cancer is presented in Figure 9.

Figure 9

Application of the in vivo diagnostic NMs in early diagnosis of cancer.

A co-precipitation-hydro/solvo/solvo-thermal method was used to synthesize [123] a novel NIF fluorescence-driven contrast medium. It produced different wavelengths of luminescence through the photoelectron transition, and further photo-generated electrons produced five bands of upconversion emission light. The enhanced density of hybridized electronic states in Ag-NaYF₄ may facilitate photon absorption and increase luminescence. The results showed that even at a low concentration (0.2 M), the contrast medium could produce green light by NIR light, while the upconversion NMs doped with Ag could increase the luminescence intensity by nearly 13 times. The in vivo experiments further demonstrated that the specific signal to tumors without outflow could be used to early diagnose the neoplasm metastasis of complex cancers via the blood vessel.

SIO NPs can be the latest contrast medium for MRI detection. Zou et al. [58] prepared mucin 1 (MUC1)-modified SIO NPs. The particle size at the nanoscale could exhibit superparamagnetism, which accelerated the dephasing process of the tissue in the local magnetic field and shortened the transverse relaxation time. The results showed that after injection of MUC1-SIO NPs, the signal in the tumor area was significantly reduced, and the area of T2-weighted signal reduction was in a concentration-dependent way. On the contrary, there were no differences in varied concentrations between the SIO NPs and bovine serum albumin SIO NP. Therefore, the MUC1–SIO NPs had the contrast-enhanced effects of MRI signal between tumor and normal tissue.

4.2 Electrochemistry

The electrochemical technique is commonly used to measure the changes of potential and current. It owns some advantages such as high sensitive, simple operation, and portability. Common analytical methods include amperometry, voltammetry, conductivity measurement, and impedance spectroscopy. Currently, different types of NMs, such as carbon-based NMs, organic NMs, and metal NPs, are prepared electrochemical sensors that can obtain electrochemical information caused by NMs by reading the transducer changes [124] (Figure 10). It has shown excellent sensing performance in terms of sensitivity and selectivity, and LOD.

Figure 10

Principle of NMs in electrochemical applications.

Peng et al. [125] designed an ultrasensitive sensor of nanocomposites with carbon nitride NMs and Au NPs. After the surface of the electrode was modified with a specific hairpin, the sensor could recognize the miRNA-155 specifically. MicroRNA-155 could form hybrid double strands with DNA, leading to the reduced transfer rate of electrons and the downregulated electrochemical signal. The experimental information confirmed the biosensor was sensitive and had good recovery when used to test miRNA-155 of serum samples.

4.3 Immunoreaction

An immunoreaction occurs when an antigen binds with an antibody, characterized by high selectivity [126] and sensitivity [127,128]. Immunosensors utilize the logic relationship between signal converters and biological components to convert biochemical signals into detectable physical signals. The biological components of the immunosensor are usually composed of antibodies or antigens. The immunosensor could sense the signal after the formation of an immune complex coupled with antigens and antibodies. Specific mutual recognition of the antigen and the antibody endows the immunosensor with better sensitivity and stability. Sensors can detect trace amounts of markers biomolecules in the cancer tissue. The immunosensors are facing broader development prospects in early diagnosis of cancer.

The construction of a high-performance immunosensor involves two major processes: the fixation of biological recognition elements and the design of probes with excellent sensing performance. The NMs can enhance the fixation of biorecognition elements onto the immunosensor because of stability and biocompatibility. By using the optical properties of NMs or simulating the enzyme activity, a stable and specific sensing probe can be prepared. Jafari et al. [129] prepared a nanosensor of Ag NP-modified GQD ink decorated with CA-125 antibody (AgNPs-GQDs) (Figure 11). The redox peak of AgNPs-GQDs nano-ink was enhanced after modification, indicating that the AgNPs-GQDs nano-ink could effectively improve the rate of electron transfer. DPV and square wave of voltammetry were used to measure the concentration of CA-125 protein in the samples. When an antibody–antigen complex was formed, the transfer of the electron was reduced, causing changes in electrochemical signals. Thus, AgNPs-GQDs can be used to develop novel nanoscale immunosensors.

Figure 11

Nanosensor of Ag NP-modified GQD ink decorated with CA-125 antibody.

4.4 Colorimetric reaction

The colorimetric method is used to determine the concentration of components by analyzing the color changes of a colored substance solution (Figure 12) [130]. The principle underlying the colorimetric reaction of NMs has two aspects. One is that LSPR causes highly restricted and enhanced electromagnetic fields on the surface of NPs, resulting in an appearance of an obvious absorption band in visible and NIF wavelengths. Given that the LSPR spectrum is affected by certain characteristics, such as the size, shape, composition, dielectric environment, and aggregation of NPs [131], the color of the solution changes dramatically. The other is based on the principle that the oxidation of dyes catalyzed by peroxidase, leading to color changes in the solution containing biomarkers [132]. Many NMs with enzyme-like properties have been applied in the colorimetric detection of cancer biomarkers.

Figure 12

Principle of NMs applied in colorimetric reaction.

Borghei et al. [133] developed a simple but sensitive colorimetric method to detect cancer cells because of aptamer–cell interaction. Selective interaction between overexpressed nucleolin receptors and aptamers (AS1411) helps the latter capture cancer cells. The AS1411 specifically bound by target cells were not hybridized with the complementary ssDNA-AuNP probe, which the solution removing all the aptamers remained red, whereas that aptamers not bound by normal cells would hybridized with ssDNA-AuNP probe, resulting in a purple solution. The colorimetric method was also combined with a paper-based biosensor based on DNA template Ag/Pt NCs to detect miR-21 in human urine samples. It could be used to accurately quantify the level of miR-21 because NCs could catalyze hydrogen peroxide and 3,3′,5,5′-tetramethylbenzidine to show a blue glow [134].