Concentration of Algal Cells for Raman Analysis by Electrowetting on Dielectrics (EWOD)

Abstract

IntroductionAmong various types of renewable energy, bio fuel is receiving more and more attention and has become one of the promising energy sources for the future energy application. Utilization of the microalgae as a source for bio fuel production offer many merits such as high lipid content, high growth rate, no need to compete with arable land [1], higher energy density [2], absorbing carbon dioxide to mitigate global warming and producing other valued compounds. In order for successful commercialization of bio fuel, one of the important steps is to be able to quantify the lipid content inside the microalgae in a rapid fashion such that screening of lipid-rich algal strains in a timely manner, optimization of the cultivation conditions and on-site monitoring of the cultivation process become feasible. For conventional techniques used to quantify the microalgae cellular lipids such as gas chromatography (GC) analysis and gram measurement [3][4][5], they are typically invasive, time-consuming, and involved in expensive instrument and well-trained personnel. The lipidconjugated fluorescent dye such as Nile red and BODIPY 505/515 was used to quantify the lipid content through fluorescence intensity analysis and similar results can be achieved compared to those from conventional techniques [6][7][8]. Raman spectroscopy, a powerful tool to determine chemical composition of materials, has been utilized to characterize many biological systems [9]. This is because the Raman signal of water is weak and uncomplicated, the sample can be analyzed in-vivo, and sample preparation is generally not required [10].Identification of algae species [11,12], probing the influence of environment on microalgae [13], quantification of degree of unsaturation [14] and β -carotene [15] in the lipid bodies, etc. have been reported. Recently, rapid quantification of lipid content inside microalgae using Raman spectroscopy has been demonstrated [16]. The algal paste was first prepared after 1-hour evaporation of an algal droplet on a gold coated glass slide, followed by applying near-infrared Raman spectrometry. The method provided representative information of a microalgae culture through cell ensemble measurements and shortened the time for quantification to less than 1.5 h. However, the coffee ring effect during droplet evaporation leads to an undesired inhomogeneous deposition of algal cells distributed across the algal paste. As a consequence, the signal intensity will vary from one location to another and searching for "hot spots" on the algal paste to provide strong signals is inevitable. The coffee ring effect is a phenomenon which results from pinning of the three-phase contact line and a convective flux driven by evaporation brings the solute to deposit in a ring at the edge [17]. Several approaches have been proposed to suppress the coffee ring effect like making the substrate surface super hydrophobic [18], generating Marangoni flow [19], utilizing the electric field [20][21][22], adjusting the viscosity and drying time of the...