Cobalt ion (Co²⁺) is mainly present in the human body to form cobalamin (vitamin B12), playing a vital role in various catalytic reactions and myelin synthesis (Tvermoes et al., 2013; Kim et al., 2017). However, excessive intake can affect the nervous system and lead to heart failure and other diseases, such as pulmonary hypoplasia and thyroid damage (Lindsay and Kerr, 2011; Hussain et al., 2018; Saqib et al., 2018). Humans mainly take in Co²⁺ through food, water, and breathing. Therefore, it is essential to develop highly efficient and sensitive strategies to monitor the concentration of Co²⁺ in different propagation mediums, considering the health risks associated with Co²⁺.

Detecting metal ions typically involves traditional methods such as inductively coupled plasma mass spectrometry (ICP-MS) or atomic absorption spectrometry (AAS) (Hanhauser et al., 2020; Hou et al., 2020; Kong et al., 2020; Lin et al., 2020; Hong et al., 2021). However, these methods require extensive technical knowledge and time-consuming sample preparation. Despite being highly sensitive and accurate, they still need high requirements and tedious pretreatment (Shirani et al., 2019; Ma et al., 2020). To address these limitations, new detection methods have been developed using fluorescent spectrometry, electro-chemiluminescence, and colorimetry, which offer high convenience and rapid response (Xiao et al., 2016; Wu et al., 2023). However, with limited methods available, Co²⁺ detection with high sensitivity and selectivity still poses a challenge.

Chiral nanomaterials possess distinctive asymmetrical configurations, and their components can be manipulated, which allows for controlling chiral optical activities in the visible or near-infrared regions. This characteristic provides excellent anti-interference and high sensitivity towards configurational changes, making it possible to detect biological molecules such as disease markers, drugs, and metal ions with high sensitivity. For example, gold mimetic nanoparticles (L/D-P⁺ NPs) synthesized by Liguang Xu et al. possess excellent immunomodulatory capabilities and can be used to regulate the maturation of immune cells, in which the L-P⁺ NPs showed a higher (800-fold) efficiency as adjuvants for H₉N₂ influenza virus vaccination than commercial aluminum adjuvants (Xu et al., 2022). However, based on these advantages, using chiral nanomaterials for metal ion detection still needs to be explored.

Metal nanoclusters (NCs) are ultramicroscopic particles with specific optical properties, such as chirality and fluorescence (Zhang et al., 2020; Zhao and Li, 2020; Zhang et al., 2021; Sagadevan et al., 2022; Li et al., 2023). They have been used to construct metal ion-responsive luminescent probes based on aggregation-induced emission (AIE) characteristics, which displayed high sensitivity and signal-to-noise ratio. For example, Deyan Qi et al. synthesized CuAuNCs-Ce assemblies by exploiting the aggregation-induced properties between Ce³⁺ and glutathione-capped bimetallic copper and gold nanoclusters (CuAuNCs@GSH). The introduction of Ce³⁺ dramatically enhanced the fluorescence properties and quantum yield of the probe, which was applied to the highly sensitive detection of ATP with a detection limit of 53 nM (Qi et al., 2022). However, the chirality response of chiral metal NCs toward metal ions has yet to attract much attention. In this study, an innovative approach for Co²⁺ detection has been established based on the chiral responsiveness of L-GSH-Au NCs toward Co²⁺ (Scheme 1).

Based on TEM images, the size of L-GSH-stabilized Au NCs (L-GSH-Au NCs) was determined to be 1.4 ± 0.3 nm (Figure 1A); the size of Co²⁺ treated L-GSH-stabilized Au NCs (L-Co-GSH-Au NCs) was analyzed to be 1.4 ± 0.2 nm (Figure 1B). After adding Co²⁺ with a final concentration of 500 μM, no noticeable morphological changes were observed in L-GSH-Au NCs. However, the CD and fluorescent (FL) optical activities of L-GSH-Au NCs changed obviously. In detail, the CD signal of L-GSH-Au NCs was initially distributed before 410 nm and shifted to the visible light region, with two new CD peaks appearing at 475 and 645 nm due to the addition of Co²⁺ (Figure 1C), accompanied by apparent changes in absorption (Supplementary Figure S1). The asymmetrical factor (g-factor) of L-GSH-Au NCs enhanced 40.22-fold after adding Co²⁺ (Supplementary Figure S2). Additionally, the FL emission peak of L-GSH-Au NCs displayed at 720 nm decreased significantly when Co²⁺ was added to the colloid solution of L-GSH-Au NCs (Figure 1D).

The mechanism of the CD and FL signal changes of L-GSH-Au NCs induced by adding Co²⁺ was further investigated. First, no obvious morphological and size changes were observed after the addition of Co²⁺, thus the aggregation of L-GSH-Au NCs can be excluded (Figures 1A, B). Therefore, it is likely that a new coordination state was formed between Co²⁺ and individual L-GSH-Au NC. This deduction could be demonstrated by the decreasing surface charge when Co²⁺ was added to the colloid solution of L-GSH-Au NCs (Supplementary Figure S3). The measurement results of X-ray photoelectron spectroscopy (XPS) further illustrated the suspicion. They suggested the presence of Co elements in the sample of L-Co-GSH-Au NCs (Supplementary Figures S4, S5; Supplementary Table S1), which meant that Co²⁺ indeed interacted with L-GSH-Au NCs. The high-resolution XPS spectra of Co 2p that were distributed in the sample of L-Co-GSH-Au NCs displayed two binding energies at 780.5 and 796.9 eV, indicating that the Co ions were presented with +2 valence (Figure 2A). The binding energy of N and O also changed significantly due to the addition of Co²⁺, suggesting the probable coordination between L-GSH-Au NCs and Co²⁺ via the surface carboxyl and amino groups (Figures 2B, C). The Fourier transform infrared spectroscopy (FTIR) spectra of L-GSH-Au NCs and L-Co-GSH-Au NCs were also measured (Figure 2D). The FTIR peaks at 1,230 and 1734 cm^-1 corresponded to C=O, and the FTIR at 1,533 and 3,081 cm⁻¹ corresponded to -NH. These FTIR peaks decreased obviously due to the addition of Co²⁺, which proved that Co²⁺ indeed coordinated with the -NH and -COOH functional groups (Shi et al., 2022; Ding et al., 2024). The -NH and -COOH functional groups originated from the GSH molecules modified on the surface of L-GSH-Au NCs via Au-S covalent bonding. According to previous studies, Co²⁺ should coordinate with two GSH molecules on the surface of Au NCs in the specific mode of CoN₂O₂, which attributed to the signal change of chirality (Ding et al., 2024).

After that, the response-ability of the CD and FL signals of L-GSH-Au NCs was investigated by adding different concentrations of Co²⁺. The results showed that the CD intensity at 475 nm and 645 nm enhanced significantly as the concentration of Co²⁺ increased from 0 to 500 μM (Figure 3A), accompanied by the absorption increase and FL intensity decrease (Figure 3B; Supplementary Figure S6A). When the concentration of Co²⁺ was increased from 500 to 1,000 μM, the CD signal at 645 nm reached a plateau (Figure 3C). However, the absorption at 368 nm continued to increase (Supplementary Figure S6B). Additionally, the FL intensity at 720 nm slightly decreased, as depicted in Figure 3D. The negligible CD signal changes indicated that Co²⁺ occupied all the covalent binding sites on the surface of L-GSH-Au NCs. The absorption and FL signal changes with the concentration of Co²⁺ ranging from 500 to 1,000 μM were attributed to the aggregation of L-Co-GSH-Au NCs induced by the excess amount of Co²⁺ (Supplementary Figures S7, S8).

In order to test the detection sensitivity of the CD and FL signals, the linear relationships between the CD intensities and Co²⁺ concentrations, as well as the FL intensities and Co²⁺ concentrations were examined. It could be observed that the CD intensity at 645 nm and FL intensity at 720 nm showed good linear relationships with the logarithm of Co²⁺ concentrations ranging from 10–500 μM, respectively (Figures 3E, F). Meanwhile, the absorption intensity at 368 nm did not display any linear relationship with the concentration changes of Co²⁺ (Supplementary Figure S6C), which was attributed to the disturbance of the natural color of Co²⁺. The limits of detection (LOD) were calculated to be 0.37 μM and 1.45 μM based on the CD and FL signals. The results indicated that the CD signal was more sensitive than the FL signal under the same detection conditions. The higher detection sensitivity of CD signals should derive from their higher sensitivity toward the chiral configurational changes compared with that of FL signals.

In addition to the detection sensitivity, the selectivity of the CD and FL signals was also studied. According to Figure 4 and Supplementary Figure S9, the FL intensity of L-GSH-Au NCs is enhanced in the presence of Ti⁴⁺, Cu²⁺ or Pb²⁺. On the other hand, GSH, BSA, Cr³⁺, Ca²⁺, Na⁺ and Cd²⁺ showed negligible effects on the FL intensity of L-GSH-Au NCs, while other metal ions resulted in decreases in FL intensity. However, among these metal ions and biological molecules, only Co²⁺ enhanced the CD signals of L-GSH-Au NCs (Figure 4; Supplementary Figure S10). These results illustrated that the CD signals of L-GSH-Au NCs exhibited high selectivity towards Co²⁺ while the FL signal did not. The high selectivity of L-GSH-Au NCs’ CD signal should originate from the specific coordination of CoN₂O₂ between Co²⁺ and L-GSH-Au NCs, which made the Co²⁺ differs from other metal ions.

Thanks to the high sensitivity and excellent selectivity of L-GSH-Au NCs’ chiral response, the Co²⁺ concentrations in various real samples, including Taihu water, animal serum, and tap water, were measured based on the CD signal of L-GSH-Au NCs. To investigate the detection ability of L-GSH-Au NCs’ chiral signal, the water samples were spiked with Co²⁺ to achieve a final concentration of 10.00 µM. As indicated in Table 1, the spiked recoveries of water samples ranged from 95.63% to 104.68%, with the relative standard deviations (RSDs) of 1.27%∼1.74% (n = 3). Moreover, the Co²⁺ concentration in the serum samples that were diluted 100-fold was spiked to be 10, 30, and 50 µM. The recoveries were detected to be ranging from 92.63% to 105.46%, with RSDs of 1.27%∼2.57%. Besides, the concentration of Co²⁺ in the real samples with spiked Co²⁺ was measured via ICP-MS. The Co²⁺ detection results from ICP-MS were consistent with those obtained from the CD assay, confirming that L-GSH-Au NCs can accurately detect Co²⁺ in real samples. Above all, L-GSH-Au NCs were proved to be a reliable and practical tool for detecting Co²⁺ in various samples.

In conclusion, this study systematically investigated how L-GSH-Au NCs’ CD and FL signals respond when interacting with various metal ions and biological molecules. This research revealed that the CD signal of L-GSH-Au NCs not only displayed high sensitivity but also showed selectivity toward Co²⁺ compared to the FL signals. This unexceptional finding led to the successful development of a convenient method for detecting Co²⁺ without the need for complex pretreatment. Based on the chiral responsiveness of L-GSH-Au NCs, the detection method could accurately quantify the Co²⁺ concentrations in water and serum samples. This study demonstrated the detection potential of nanomaterials’ CD signals and introduced a new approach to developing highly sensitive and selective detection probes suitable for complex detection scenarios.