**4. Conclusions**

MPs, as an environmental pollutant, have become a global problem and they may pose a risk to human health. To summarize research progress and identify future research topics based on current hotspots, we conducted a bibliometric profile of MPs relevant research, using data from the WoSCC database for the period 2004–2019. We found that the scientific output of MPs-related research experienced rapid growth during the past 16 years, and that this booming research area has expanded into many related fields. Developed countries were important contributors to MPs research, as England, the USA, and Germany occupied the top three positions, based on the criterion of NSC. China was the only developing country in the top 10 national contributors. These influential countries foster close academic collaborations, as shown by the co-authorship network analysis of countries. However, more exchanges and cooperation between these countries and others are needed. We also found that Thompson RC, who defined the term "MPs", was the most productive author, as well as the most influential one. All of the top 10 authors identified based on the criterion of NSC were ESI highly cited researchers in the *Ecology*/*Environment* field, which highlights their significant influence on MPs-related research. Our results show that the issue of MPs is a multidisciplinary research field, because journals classified in six different categories contained MPs-related articles. The results of co-citation network analysis of cited references indicated that in-depth research laid a solid foundation for the MPs scientific field. The internal composition relationships of MPs studies were visualized by co-occurrence networks of author keywords, both in extracted publications and ESI highly cited papers, which identified the following research hotspots: potential sources and spatial distributions of MPs, analytical methods, the interaction of MPs with contaminants, and the impacts of MPs on organisms as well as human beings. Future MPs studies should focus on the following five aspects: (1) integration of MPs monitoring into existing environmental monitoring programs; (2) unified technical standards and reliable quantification routines; (3) sorption/desorption behavior of contaminants on MPs in different

environments; (4) biological effects on sentinel species and molecular toxicology mechanisms; and (5) the effects of MPs exposure on human health.

**Author Contributions:** Conceptualization, F.Q., J.D. and H.W.; methodology and software, F.Q., J.G. and G.L.; formal analysis, J.D., J.G., G.L. and Y.S.; investigation, A.Y., Y.D. and Q.W.; resources, J.G., G.L. and H.W.; writing—original draft preparation, review and editing, F.Q., J.D. and H.W.; visualization, F.Q., Y.S. and A.Y.; funding acquisition, J.D. and H.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by The Science and Technology Innovation Fund of Dalian, P.R. China, grant number 2019J13SN119; Key Laboratory of Huanghuai Water Environment and Pollution Control, Ministry of Education, P.R. China, grant number KFJJ-2017-11; Department of Ocean and Fishery of Liaoning Province, P.R. China, grant number 201808.

**Acknowledgments:** We would like to thank XianWen Wang of Dalian University of Technology for his technical support on VOSviewer software.

**Conflicts of Interest:** The authors declare no conflict of interest.
