*2.1.2 The sensing device*

The Lab On A Disc (LOAD) centrifugal microfluidic platform that uses a centrifugal driving force. To date, on-chip water quality assessment systems have been primarily developed using Lab On A Chip (LOAC) systems, with only a few examples reported on centrifugal disc (CD) platforms [20, 21]. The EU Framework 7 project, MariaBox (www.mariabox.com) set out to develop an autonomous sensor for marine application with the ambitious aim of autonomous operation for several months in the field. The biosensor disc was developed at DCU. The original biosensor concept was to enable the determination of eight analytes (including marine toxins domoic acid, saxitoxin; poly aromatic hydrocarbons and metals) on-chip simultaneously, with each requiring a different biological or chemical assay method. The final 8-analyte sensor disc evolved through development and testing and emerged as an effective design enabling different bio- and chemical assays to be performed on the same chip by using antibodies, valving, and optimal micro channel design.

The complexities in developing a multi-analyte centrifugal microfluidic platform are many. These technical iterations are summarized in **Figure 2a** as well as the incremental optimization practices that were employed. The process starts with an initial concept, biosensor assay integration with material surface modifications, microvalve development for synchronised assay execution, and the final deployable outputs.

There are multiple advantages associated with LOAD platforms including; autonomous pumping using centrifugal forces, precise liquid handling, control of samples using valves, ability to multiplex assays using identical test conditions and a myriad of detection techniques compatible on disc, making it an ideal technique for *in-situ* environmental monitoring. The LOAD consists of a multi-layer disc that when combined, holds the channels, valves and reservoirs (**Figure 2b**).

To develop the centrifugal microfluidic platform (**Figure 2a**), a number of factors were considered; sample/reagent type/size, analyte characteristics, sample matrix, assay protocols, transduction method, targeted style of use (autonomous, handheld), usage (single, multiuse). The detection strategy on centrifugal microfluidics, offers a myriad of transducing solutions for biosensing [14, 22]. The key to developing a successful, multi-analyte environmental processing unit lies within the integration and adaptation of new and current microfluidic features and transducing elements in microfluidic platforms [23]. With further development, this type of biosensor for multiple analyte determination can transform how data can be gathered from hard to reach or remote marine or surface water environments in real-time or near real-time.

LOAD devices allow for rapid on-site measurements, with miniaturisation and automation of laboratory-based analytical protocols, towards the development of inexpensive, portable, and compact devices [17]. The use of microlitre volumes results in a reduction in reagent consumption and waste production compared to laboratory protocols. This, coupled with reduced cost and lowered sample contamination risk compared with standard methods for water analysis makes disposable microfluidic devices an attractive option [16]. The ease with which discs can be modified and built means that bioassays can be readily transferred to sensor systems.

## **2.2 Hand-held phosphate analyser for freshwater monitoring**
