## 3 Results
### Optimization of the SERS Measurement Conditions  
Using metallic nanoparticles in an aqueous solution for the SERS measurements, issues associated with batch-to-batch compatibility and aging effects are well known. Thus, to prevent the mentioned obstacles an in situ approach for the AgNP production is suitable. For this propose, the microfluidic platform, established by [PERSON] et al. [42], provides the required preparation conditions. A constant volume flow allows for very precise mixing ratios. This is combined with the possibility to mix and add substances separately, ensuring for perfect mixing conditions. Furthermore, the long measuring unit enables variation of the reaction time in order to optimize the signal intensity. For the in situ production of AgNPs in a microfluidic chip device, the Lee-Meisel protocol [54] is not suitable because of the synthesis requirements of boiling under reflux conditions for over one hour. The Leopold-Lendl protocol [46] instead can be realized under room temperature conditions and on a time scale shorter than one minute [55]. However, the required sodium hydroxide would react with the hydrophobic silane layer on the glass surface of the microfluidic chip and at the same time inhibit the successful creation of droplets as well as the nanoparticles. As an alternative method, hydrazine can be used as reduction agent in the presence of a low amount of citrate to synthesize y-AgNPs in the applied microfluidic chip device [47].  
When large batch protocols are transferred to microfluidic platforms new degrees of freedom, such as flow rates or time elapsed from substance mixing to measurement, arise. Thus, the production protocol needs to be adapted in order to control the order of addition and the concentration of the reagents in the mixture. As POA is the main targeted analyte, the influence of the flow rates and the addition of citrate as a stabilizing agent were investigated in order to establish the ideal measurement conditions. Generally, a proper and reproducible mixing of reagents during in situ colloid synthesis is essential to achieve reliable SERS results. In order to synthesize the hydrazine reduced silver NPs in the microfluidic chip three initial reagents are needed: An aqueous solution containing 3 mM AgNO\({}_{3}\) and 15 mM NH\({}_{3}\) (flowrate: 5-30 nL/s), an aqueous solution of 1.5 mM N\({}_{2}\)H\({}_{4}\) (flowrate: 5-30 nL/s) and an aqueous solution of the stabilizing agent sodium citrate with the concentration of 10\({}^{-5}\) M (flowrate: 5 nL/s) are inserted to the chip at the droplet generator unit. For the experiment the flowrate of mineral oil (separation media) was varied between 5 and 30 nL/s, while the flowrates of POA and the aggregation agent (0.5 M Na\({}_{2}\)SO\({}_{4}\)) were kept constant at 10 nL/s each. The mean of the measured spectra for the varying flowrates are depicted in Figure 2.  
It was found, that the optimized environment for a good signal to noise ratio and high intensity of the SERS spectra is achieved when citrate was added (5 nL/s) and the flowrates of mineral oil, AgNO\({}_{3}\)/NH\({}_{3}\) solution and N\({}_{2}\)H\({}_{4}\) was set to 30 nL/s. This might be attributed to better mixing conditions of the reagents due to the increased flow velocity. In a next step, after the reduction of silver nitrate was completed, the analyte was injected into the droplet through the dosing unit and pure SERS spectra of the analyte molecules were recorded with a very good signal-to-noise ratio. In general, a triple determination is required to achieve a reliable date set. Subsequently, three spectra were measured in every droplet (integration time 0.5 s) and the average spectrum was estimated.