Blue micro-/nanoplastics abundance in the environment: a double threat as a Trojan horse for a plastic-Cu-phthalocyanine pigment and an opportunity for nanoplastic detection via micro-Raman spectroscopy†

Abstract

Blue plastics, whether macro- or micro-sized, are intriguingly frequently reported in significant numbers of studies dealing with the contamination of environmental waters or living organisms with microplastics. In our recent investigations on microplastics in environmental waters, we noted abundant blue microfibers and fragments, whose identification was achieved via Raman spectroscopy. Still widely used in the plastics industry, despite awareness raised at a global level, copper phthalocyanine (CuPc), a blue pigment carried by abundantly used blue plastics, like a “Trojan horse”, is a secondary threat (after plastics) in the trophic chain, being highly resistant to a broad range of conditions. Here, the newly discovered resonance Raman (RR) signal of the blue pigment CuPc embedded in environmentally aged plastics allowed us to lower the minimum size of detectable nanoplastics via micro-Raman spectroscopy down to 500 nm. In addition, we demonstrated nanoplastics detection solely via the CuPc RR signal, a result subsequently validated using SEM–EDX. Based on a visible-NIR Raman spectroscopy investigation of isolated synthetic CuPc compounds, we discussed the observed changes in pigment spectra in blue plastic waste aged for an estimated 20 years, consisting of highly brittle ropes knotted in fishing nets and harvested via scuba diving from the seabed. Thus, the fate of CuPc in environmentally aged blue plastics could provide robust analytical opportunities when studying the impact of aged, blue-coloured plastics at various levels, due to its persistent, selective and specific RR signal. The results are crucial for expanding the capability of Raman tools for further tracking the micro-to-nanoplastic degradation of waste along the trophic chain, improving our understanding of its impact on living organisms.