Accurate assays for the detection of antibodies to SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) are essential for the control of the COVID-19 (coronavirus disease 2019) pandemic. Here, we report antibody and antibody-avidity assays, relying on near-infrared-fluorescence amplification by nanostructured plasmonic gold substrates, for the simultaneous detection of antibodies to the S1 subunit of the spike protein and to the receptor binding domain of SARS-CoV-2 in human serum and saliva, and for quantifying immunoglobulin avidities against coronavirus antigens from SARS-CoV-2, SARS-CoV-1 and the common-cold viruses OC43, HKU1, NL63 and 229E. The antibody assay detected immunoglobulin M in 87% (52 of 60) COVID-19-positive serum samples collected 6 or more days after symptom onset (and the immunoglobulins M and G in all 33 samples collected at least 15 days after symptom onset), and correctly classified 456 out of the 457 COVID-19-negative serum samples tested (424 of them collected before the pandemic, including 73 that were positive for other viruses). We used the antibody-avidity assay to study antibody-maturation patterns, anamnestic responses, and cross-immunity to the common-cold coronaviruses.

CO 2 dissolved in aqueous solutions CO 2 (aq) is important to CO 2 capture, storage, photo-/electroreduction in the fight against global warming and to CO 2 analysis in drinks. Here, we developed microscale infrared (IR) spectroscopy for in situ dynamic quantitating CO 2 (aq). The quantized CO 2 (g) rotational state transitions were observed to quench for CO 2 (aq), accompanied by increased H 2 O IR absorption. An accurate CO 2 molar extinction coefficient ε was derived for in situ CO 2 (aq) quantification up to 58 atm. We directly measured CO 2 (aq) concentrations in electrolytes under CO 2 (g) bubbling and high-pressure conditions with high spectral and time resolutions. In KHCO 3 electrolytes with CO 2 (aq) > ~1 M, CO 2 electroreduction (CO 2 RR) to formate reached >98% Faradaic efficiencies on copper (Cu 2 O/Cu)–based electrocatalyst. Furthermore, CO 2 dissolution/desolvation kinetics showed large hysteresis and ultraslow reversal of CO 2 (aq) supersaturation in aqueous systems, with implications to CO 2 capture, storage, and supersaturation phenomena in natural water bodies.