## 1 Introduction
Some examples of these referenced devices are: the FluoroProbe II from bbe Moldaenke, which weighs 4.6 kg and costs around EUR 23,000; the Algae Online Monitor from Photon Systems Instruments, which weighs 3.4 kg and costs around EUR 12,990; and the AlgaeTorch from bbe Moldaenke, which weighs 1.2 kg and costs around EUR 7500 [16]. Therefore, development of new methodologies and novel devices, which can offer cost-effective, high-throughput and non-destructive analysis of microalgae species, has attracted increasing interest from research communities. Recent works have reported on the development of portable sensors that measure different phytoplankton species through fluorescence detection of both the primary photosynthetic pigment (chlorophyll a) and accessory pigments (such as chlorophyll b, caroten, phycocyanin, phycerythrin, and others) which are present in microalgae and cyanobacteria. Among these recently developed portable sensors, some can be highlighted, such as the development of a hand-held fluorescence sensor platform capable of selectively estimating green algae and cyanobacteria biomass [14]. In other works, the optofluidic detection systems are presented according to their category, such as fluorescence sensing, Raman spectroscopy and imaging flow cytometer, where they compare and discuss their advantages and disadvantages and identify avenues of further development of microfluidic systems for microalgal detection and characterization [17]. With an increasing demand for fluorescent portable sensors due to the benefits that fluorescent sensing technology offers, an overview of recent developments of portable fluorescence sensors can be found in [9; 18]. However, most of them only differentiate different mixtures of green algae and cyanobacteria, at a higher taxonomic group. In [19], the authors report a device featuring similar characteristics to the herein presented. However, despite its low cost and small dimensions, their device, as presented, is not a stand-alone system. In contrast, the system hereby presented aims to be integrated in a fully autonomous and portable lab-on-a-chip for in situ toxic phytoplankton quantification. To our knowledge, there is not any methodology yet allowing the quantification and identification of phytoplankton groups at the same time, that also features low cost, portability, is prepared for continuous underwater monitoring and is autonomous. Therefore, we present an identification methodology for inclusion in a portable lab-on-a-chip, based on:  
1. Multi-wavelength LED lighting excitation;
2. Photodiode based multi-wavelength fluorescence detection;
3. Scattering and absorption optical properties;
4. Single-cell analysis. Phytoplankton has specific optical characteristics, which naturally absorbs light in the visible spectral range (mainly in blue light) and fluoresces in the 600 nm to 750 nm spectral range. Moreover, phytoplankton cells have different pigments, and each pigment has its own absorbance and fluorescence characteristics. Each phytoplankton group also have different cell sizes and complexity, which translates in different reflection, transmission and diffraction of light, as previously reported by flow cytometry [20]. Considering these optical properties, it is possible to identify and quantify the different groups of phytoplankton in aquatic systems [6]. Moreover, a single-cell analysis system is necessary to avoid the averaging effect when several cells are analysed at same time. It should be noted that device miniaturization efforts emerged as a potential alternative, introducing improvements on efficiency, portability, automation, analysis time and reagents/samples reduction, as well as a reduction in costs [21]. In this work, the spectral characteristics of three species (_Nannochloropsis gaditana, Isochrysis galbana_ and _Tetraselmis suecia_) were studied using benchtop flow cytometry, commercial spectrophotometers and fluorimeters.