6 mercaptohexanoic acid

6-mercaptohexanoic acid (MHA), were purchased from Sigma-Aldrich. Hydrogen tetrachloroaurate(III) hydrate (HAuCl₄·3H₂O) was purchased from Alfa Aesar. Sodium hydroxide (NaOH) was purchased from Sigma-Aldrich. Carbon monoxide (CO, 99.9%) was obtained from Singapore Oxygen Air Liquide Pte Ltd (SOXAL). All chemicals were used without further purification. Ultrapure water (18.2 MΩ cm) was used in all the experiments. All glassware was washed with aqua regia before use.

Myoglobin extracted from horse heart (where a metalloprotein is used to transfer oxygen using the ionic state change of iron ions) was purchased from Sigma-Aldrich (St. Louis, MO., USA). 2-mercapto acetic acid, 3-mercaptopropionic acid, 6-mercaptohexanoic acid, 8-mercaptooctanoic acid, 11-mercaptoundecanoic acid, 16-mercaptohexadecanoic acid, cysteamine hydrochloride, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), N-hydroxysuccinimide (NHS), and rhodamine B were also purchased from Sigma-Aldrich. The developer Su-8 2 and the remover PG were purchased from Microchem (USA). 10 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) with pH 7.0 adjusted by sodium hydroxide (NaOH) and hydrochloric acid (HCl) was used for the preparation of protein solution. Distilled and deionized (DI) water with a resistance of less than 18 MΩ produced by a Mili-Q system was used in all experiments to prepare the chemical solution and to rinse the substrate.

1-Azidoundecane-11-thiol and Hemin-yne were synthesized following the reported procedure29 (link)69 (link). 6-Mercaptohexanoic acid was purchased from Sigma Aldrich. Di-sodium hydrogenphosphate dihydrate (Na₂HPO₄. 2H₂O) was purchased from Merck. 2, 6-lutidine was purchased from Avra Synthesis Pvt. Ltd. These chemicals were used without further purification. Au wafers were purchased from Platypus Technologies (1,000 Å of Au on 50 Å of Ti adhesion layer on top of a Si(III) surface). Transparent Au wafers (100 Å of Au on 10 Å of Ti) were purchased from Phasis, Switzerland. Au and Ag discs for the RRDE and SERRS experiments, respectively, were purchased from Pine Instruments, USA. The Mb mutants were prepared as reported in the literature7 (link)30 . Analysis of the components of the rR spectrum was done by using Lorenztian line shape of peak fit software.

Biotin-labelled cyclic citrullinated peptide (Biotin-CCP) (4166 IU/mL) (Peptide sequence: Biotin-HQCHQEST-Cit-GRSRGRCGRSGS-COOH) with a disulfide bond between the Cyc3 and Cyc16 residues was procured from Peptron Inc. (Daejon, South Korea). Anti-cyclic citrullinated peptide antibody (Anti-CCP-ab) (4160 IU mL⁻¹) was procured from Bioss Antibodies Inc. (Woburn, Massachusetts, USA.). 6-Mercaptohexanoic acid (MHA), Hemoglobin, Potassium ferrocyanide (K₄[Fe(CN)₆]·3H2O), potassium ferricyanide (K₃[Fe(CN)₆]), 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC), N-Hydroxysuccinimide (NHS), Avidin from egg white (≥10 Units/mg protein, ≥98%), Bovine serum albumin (BSA), and human serum were purchased from Sigma-Aldrich (USA). Anti-Interferon-γ-antibody (Anti-IFN-γ-ab), Anti-insulin-antibody (Anti-insulin-ab), IgM-Rheumatoid factor (IgM-RF), Anti-C-reactive-protein-antibody (Anti-CRP-ab), Human-Immunoglobulin-G-antibody (Human-IgG-ab) were procured from Abcam (Cambridge, UK). Phosphate buffered saline (PBS- 137 mM NaCl, 2.7 mM KCl, 4.3 mM Na₂HPO₄, and 1.4 mM KH₂PO₄; pH 7.4) and 1 × PBS containing 0.05% Tween 20 were procured from Tech and Innovation (Gangwon, South Korea). All solutions were prepared using deionized water (DI; 18.2 MΩ cm) from Purescience (Jungwon, South Korea). All other chemicals were of analytical reagent grade and are used without any further purification.