Materials 4-arm poly(ethylene glycol) amine (4-PEG-NH2) (MW = 10 kDa) were purchased from JenKem Technology USA, Inc. (Allen, TX). Boc-histidine(trt)-OH (BOC), BOP reagent (BOP), N-Ndiisopropylethylamine (DIPEA), dichloromethane (DCM), trifluoroacetic acid (TFA), triisopropylsilane (TIPS), MOPS buffer (MOPS), diethyl ether, and methanol were purchased from Milipore Sigma. Synthesis of 4-PEG-histamine 4-PEG-histamine was synthesized following the method in the reference. 4 g of 4-PEG-NH2, 1.19 g of Boc-His(Trt)-OH, 1.06 g of BOP were dissolved in 15 mL of DCM in a Schlenk flask under continuous nitrogen gas flow. 0.86 mL of DIPEA was added to begin the reaction under magnetic stirring, which ran overnight under N2. The product was precipitated in 50 mL of ether, purified in 50 mL of methanol three times (by dissolving in methanol, precipitating the product in the freezer, and then separating the supernatant via centrifugation), and precipitated again in 50 mL of ether. 95 mL of TFA and 2.5 mL of TIPS were added to the solid product, and the solution was mixed for 2 h under magnetic stirring. The solvent was removed using a roto-vap, and the solid product was precipitated in 50 mL of ether three times, after which the final product was vacuum dried. The final product is a white solid, with the histidine residue exhibiting 1 H-NMR peaks (300 MHz, CDCl3) at ca. 7.45, 8.5, and 8.65, 20 allowing us to calculate an average coupling efficiency of 88%.