*3.2. Synthesis*

General. All chemicals and solvents were obtained as reagent grade from commercial sources unless otherwise stated. *Trans*-Cyclooctene-NHS ester and Tetrazine-PEG5-NHS ester were purchased from Click Chemistry Tools (Scottsdale, AZ, USA). Rituximab (MabThera®, Roche Pharma AG, Grenzach-Wyhlen, Germany) was a kind gift from the University Hospital of Innsbruck. The fluorescent dyes, SulfoCyanine5-NHS ester and SulfoCyanine7 carboxylic acid were obtained from Lumiprobe GmbH (Hannover, Germany) while IRDye800CW carboxylic acid was purchased from LI-COR Biosciences GmbH (Bad Homburg, Germany).
