You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

Differential propensity of citrate- and polyethylene glycol-coated silver nanoparticles to bovine hemoglobin. Propensity of two different silver nanoparticles (Ag-NPs) to bovine hemoglobin (BHb) was investigated by means of spectroscopic methods. We have combined spectrophotometric and calorimetric methods to show that there is no significant interaction between citrate-coated Ag-NPs and BHb at physiological pH and 20°C. However, our previous results show that polyethylene glycol-coated Ag-NPs strongly bind to Hb and effect on the secondary and tertiary structures of BHb. Thus, a suitable surface coating and modification of surface charge would increase the NPs safety and reduce adverse biological responses.
{"Chemical": ["polyethylene glycol", "silver", "citrate", "Ag"]}