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Monoclonal antibodies (mAbs) may be used as biopharmaceuticals to treat various diseases, ranging from oncology to inflammatory and cardiovascular affections. Trustworthy analytical methods are necessary to study their pharmacokinetics, both during their development and in post-marketing studies. Because biopharmaceuticals are macromolecules, ligand-binding assays (both immunoassays and bioassays) are methods of choice to measure their concentrations. Immunoassays are based on the capture of biopharmaceuticals by their target, which may be a circulating or membrane antigen or by an antibody recognizing their structure. Bioassays measure the activity of the biopharmaceutical in a specific in vitro test. A number of techniques have been reported, but their limits of detection and quantification vary widely. Anti-drug antibodies (ADA) against biopharmaceuticals are often formed and sometimes interfere with clinical efficacy. Accurate and reliable detection of ADA is therefore necessary. Binding of ADA is dependent on affinity and avidity, which makes quantification challenging. In this review, we discuss the benefits and limitations of each method to determine mAb levels and carefully compare ADA assays.

Surface-enhanced Raman scattering (SERS) is an ultrasensitive spectroscopic technique that can identify materials and chemicals based on their inelastic light-scattering properties. In general, SERS relies on sub-10 nm nanogaps to amplify the Raman signals and achieve ultralow-concentration identification of analytes. However, large-sized analytes, such as proteins and viruses, usually cannot enter these tiny nanogaps, limiting the practical applications of SERS. Herein, we demonstrate a universal SERS platform for the reliable and sensitive identification of a wide range of analytes. The key to this success is the prepared “slot-under-groove” nanoarchitecture arrays, which could realize a strongly coupled field enhancement with a large spatial mode distribution via the hybridization of gap-surface plasmons in the upper V-groove and localized surface plasmon resonance in the lower slot. Therefore, our slot-under-groove platform can simultaneously deliver high sensitivity for small-sized analytes and the identification of large-sized analytes with a large Raman gain.

A novel, microsphere-based fluorescence immunochromatographic assay (MFIA), of higher sensitivity and faster detection than the monoclonal-based competitive indirect enzyme-linked immunosorbent assay (ci-ELISA), was developed for the routine surveillance of cefalexin antibiotic residues in plasma, milk, muscle and liver. The cross-reactivities of the fluorescent microsphere–monoclonal antibody (FM–mAb) conjugates to cefaclor, cefadroxil, cefradine and cefixime were 212.7%, 155.6%, 108.1% and 86.6%, respectively. The limits of detection of the proposed MFIA were <0.1 μg L−1in all the cases, with IC50values in the range from 0.59 to 0.71 μg L−1. The recoveries were 84.5% to 107.2% with the coefficient of variation less than 6.9% when cefalexin standards were spiked in selected biological matrices. Good correlations among the MFIA, ci-ELISA and ultra performance liquid chromatography-mass spectrometry/mass spectrometry (UPLC-MS/MS) results for blood samples from a cefalexin-injected rabbit were observed. In summary, the entire assay took about 30 min, and was simple, rapid, highly sensitive and cost-effective for both high-throughput screening and individual testing in food quality control.