The paper entitled "A single drop histamine sensor based on AuNPs/MnO2 modified screen-printed electrode" has presented the novel approach for histamine detection using three electrodes screen-printed electrode system. It is a very well-structured paper. The chosen title presents a great call for the readers, and the abstract presents an overview of the entire paper, with background information, the research aim, and a summary of the proposed method. It provides an easy flow of reading from the introduction to the conclusion. The references are well structured, listed, and current (the paper was received in November 2019).

The modification of the sensor with Au/MnO2 provides better visualization of the histamine oxidation. And, this "Proof-of-Concept" research paper has shown its potential as a promising alternative for marine food quality.

Only minor suggestions and points:

1. Please, double-check some references:
a. In the second paragraph of the introduction, the first sentence "Up to know, a high number of different techniques have been employed for the determination of histamine in food samples." Inducing the reader to infer that the following references are related to the histamine determination in food samples. Maybe it should be rewritten once the closest reference [5] is related to extracellular histamine concentration in the hypothalamus of rats.

b. In the same paragraph, the sentence "...while capillary zone electrophoresis method was developed by Vitali et al. [8–10]"should be rewritten because reference [8] id Vitali et al. but [9-10] references belong to two different research groups.

c. Followed by the sentence "Due to the high importance of this compound, electrochemists developed various methods for the determination of histamine levels in food samples [11–17]." Please, check the references [12], [13], [15], and [17] because they are not tested in food.

2. The authors presented excellent discussion development over the optimization process and results. Nevertheless, I have missed the developing discussion about the interferences presented by dopamine and Vitamin B6. Once the discussion and results are in the same session, the issue should be exploited here. Why may dopamine and vitamin B6 interfere in the electrochemical response? Also, other biogenic amines (BAs) are present with histamine in marine food contamination. The authors should test the interference of BAs.

3. The authors emphasized keywords as "marine food quality" and their synonymous. In total, they appear fifteen times in the paper. Why was the method applied in human blood plasma instead of marine food samples? Besides, the real sample tested is missing from the abstract, and the reader may infer that the real sample will be marine food.

4. For human blood tests, did the authors have human ethics approval? I would suggest adding it in the acknowledgments.