Gold nanoparticles (AuNPs) are nanometre-sized gold particles with different optical, electronic, chemical, and magnetic properties depending on their size and shape [1,2,3,4]. Regarding optical properties, surface plasmon resonance causes optical absorption and light scattering in the visible light region, resulting in attractive colour tones actively applied to molecular sensors [5,6,7,8]. Furthermore, in AuNP analysis, light scattering has attracted considerable attention because it can identify changes in optical properties with higher sensitivity than optical absorption [9].

Dark field microscopy (DFM) can observe the scattered light of a single AuNP particle as a single bright spot and has been used to detect various molecules, such as DNA and proteins, by imaging the differences in the optical properties of single molecules [10,11,12,13,14]. Recently, an example of high-sensitivity measurement using a digital immunoassay was reported, taking advantage of the characteristics of dark-field imaging, in which a single AuNP particle can be observed as a single bright spot [15]. A digital immunoassay is a method for detecting targets by single-molecule measurements of nanoparticles using DFM, utilising a conventional sandwich immunoassay with captured antibodies modified on the surface of beads and labelled antibodies modified with nanoparticles. The advantage of digital single-molecule measurements is that the presence or absence of a target can be measured as a 0 or 1 signal, which is more reliable than the analogue method of measuring the signal intensity. Until now, digital immunoassays have been performed using sandwich immunoassays, which require a target antigen of a certain size that can bind to two antibodies [16,17,18]. However, antigens measured in the immunodiagnostic field include small molecules, such as thyroid and steroid hormones [19,20,21,22]. Although these small molecules are normally detected by competitive methods, this method, which uses inhibition of the antigen–antibody reaction, has been difficult to use in digital immunoassays.

In this study, we developed a digital immunoassay method using anti-immunocomplex antibodies to detect small molecules via single-molecule observation of AuNPs using DFM. Anti-immunocomplex antibodies specifically recognise immunocomplexes of small molecules with antibodies [19, 23,24,25]. Here, estradiol (E2) was employed as a model target small molecule, and an anti-immunocomplex antibody that recognises the complex of E2 and an anti-E2 antibody were used. E2 is one of the estrogen steroid hormones, and monitoring of the circulatory E2 is important in medical applications [26]. E2 has also been considered as one of the endocrine disrupting compounds due to its frequent occurrence and its effect on the environment and human health [7, 27].

The proposed digital immunoassay using DFM and anti-immunocomplex antibodies is shown in Scheme 1. Magnetic microparticles were modified with an anti-dinitrophenyl mouse monoclonal antibody (anti-DNP antibody), and the surface of the AuNPs (60 nm) was modified with an anti-immunocomplex antibody. Anti-immunocomplex antibodies were obtained by culturing the anti-immunocomplex antibody-producing hybridomas. The anti-DNP antibody-modified magnetic microparticles were immobilised with a DNP-labelled anti-E2 rabbit monoclonal antibody. E2 was added to form an immunocomplex, followed by a reaction with anti-immunocomplex antibody-modified AuNPs and bound/free (B/F) separation using a magnet to remove excess antibody-modified AuNPs. Immunocomplexes containing AuNPs formed on the surface of the magnetic particles were denatured and separated by urea and heat. The magnetic particles were removed using a magnet. The released antibody-modified AuNPs were immobilised on glass slides and observed using DFM. The bright spots obtained by DFM were evaluated using ImageJ software to obtain the number of AuNPs corresponding to the E2 concentration [28].

Scheme 1
scheme 1

Schematic illustration of E2 detection by digital immunoassay using an anti-immunocomplex antibody and DFM imaging

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A conventional immunoassay (Fig. 1a) was performed using an alkaline phosphatase (ALP)-labelled anti-immunocomplex antibody and the fluorescent substrate 4-methylumbelliferyl phosphate (4-MUP). As shown in Fig. 1b, the fluorescence intensity increased in a dose-dependent manner with increasing E2 concentration, supporting E2 detection using an anti-immunocomplex antibody. There was a significant difference (p < 0.001) between the control sample without E2 and the sample with 1 pg/mL E2, indicating that the target molecule can be detected with a concentration as low as 1 pg/mL.

Fig. 1
figure 1

E2 detection by fluorescence detection system using an anti-immunocomplex antibody. a Schematic illustration of E2 detection by fluorescence detection system. b E2 detection by the fluorescence detection system. Error bars indicate the standard deviation of five measurements. ***, p-value < 0.001

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Next, E2 detection was performed using a digital immunoassay (Scheme 1) with anti-immunocomplex antibody-modified AuNPs as the labelled antibody and DFM. As shown in Fig. 2a, DFM observed AuNP spots at the single-molecule level. The number of AuNP spots obtained from the DFM images increased in a dose-dependent manner with increasing E2 concentration (Fig. 2b). There was a significant difference (p < 0.001) between the control sample without E2 and the sample with 1 pg/mL E2, indicating that the current approach can be used to detect E2 with a concentration as low as 1 pg/mL. This result supports the successful detection of E2 using a digital immunoassay with an anti-immunocomplex antibody.

Fig. 2
figure 2

E2 detection by a digital immunoassay system using an anti-immunocomplex antibody. a E2 detection by bright spot observation using DFM. Scale bars represent 10 µm. b E2 detection by a digital immunoassay system. Error bars indicate the standard deviation of five measurements. ***, p-value < 0.001

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In conclusion, the results support the possibility of digital immunoassays for small antigens using anti-immunocomplex antibody-modified AuNPs and the single-molecule observation of AuNPs using DFM. The advantage of the digital immunoassay is that molecules could be detected with a high sensitivity with a combination of magnetic separation and counting of AuNPs at a single molecule level [15]. The current detection limit of the digital immunoassay was similar to that of conventional immunoassay methods. However, the detection limit is expected to be improved by reducing the background noise, possibly because of the non-specific binding of antibodies modified on AuNPs and magnetic nanoparticles. Further studies are necessary to inhibit such non-specific binding by optimising the blocking reagents and washing processes during B/F separation. Since various anti-immunocomplex antibodies against small molecules such as morfines, mycotoxins, peptide hepatoxins and organophosphate have also been developed [29,30,31,32], the current study would contribute to expanding target antigens in digital immunoassays using anti-immunocomplex antibodies.