Electrolyte-Gated Organic Field-Effect Transistors for Quantitative Monitoring of the Molecular Dynamics of Crystallization at the Solid–Liquid Interface

Quantitative measurements of molecular dynamics at the solid–liquid interface are of crucial importance in a wide range of fields, such as heterogeneous catalysis, energy storage, nanofluidics, biosensing, and crystallization. In particular, the molecular dynamics associated with nucleation and crystal growth is very challenging to study because of the poor sensitivity or limited spatial/temporal resolution of the most widely used analytical techniques. We demonstrate that electrolyte-gated organic field-effect transistors (EGOFETs) are able to monitor in real-time the crystallization process in an evaporating droplet. The high sensitivity of these devices at the solid–liquid interface, through the electrical double layer and signal amplification, enables the quantification of changes in solute concentration over time and the transport rate of molecules at the solid–liquid interface during crystallization. Our results show that EGOFETs offer a highly sensitive and powerful, yet simple approach to investigate the molecular dynamics of compounds crystallizing from water.


Fabrication of the EGOFETs for in-situ measurements
The interdigitated electrodes were cleaned with UV/Ozone treatment for 15 min and then treated with PFBT (1 h incubation in 10 mM PFBT solution diluted in IPA) to decrease the injection barrier that exists between gold and DPPTTT 1 . After these treatments, a solution (8 mg/mL) of DPPTTT in DCB was spin coated onto the patterned source-drain electrodes. The obtained film was annealed at 140 °C for 40 min to fully remove the solvent. The gold wire with a diameter of 0.5 mm, used as top gate, was treated with PFDT (1 h incubation in 10 mM PFDT solution diluted in IPA). A droplet (2 μL) of water or different concentration of glycine solution was placed on the substrate and the PFDT treated gold wire was immersed into it for the measurements.

Integration with the microfluidic setup
A double-sided tape with microfluidic channels laser written into it was stuck on top of the sourcedrain substrate coated with DPPTTT as described above. An array of 1 mm 2 gold electrodes was patterned onto a second PEN substrate and this was attached to a poly(methyl methacrylate) carrier. This assembly was then stuck to the second surface of the double sided tape to close the device. Holes in the substrate of the gate electrode substrate allowed water or glycine solution to enter the microfluidic channels. A syringe pump was used to control the flow rate of the solution.

Electrical measurements
These were conducted by using an Agilent B1500A semiconductor parameter analyzer in conjunction with a custom made probe station. All measurements were performed in an ambient environment at room temperature. For the EGOFETs with microfluidic device, water or glycine solution was first pumped into the microfluidic channel with a flow rate of 100 µL min -1 for 5 minutes to ensure it fully covered the channel and to remove any contaminants in the system before starting the measurement.

Section S2. Fundamentals of the EGOFET
By applying a voltage between the source and drain terminates, the charges accumulated in the DPPTTT starts flowing, by generating IDS as following: 2 where W is the channel width, L is the channel length, μ is the carrier mobility of the DPPTTT layer, Ci is the total capacitance of the EDLs, VG is the gate voltage, VTh is the threshold voltage.
To extract the values of μCi, and VTh, the square root of the IDS is plotted against VG as: μCi can therefore be extracted by the slope of (IDS) 0.5 vs VG by linear fitting of the data. The extrapolated intercept of this straight line with the VG axis is taken as VTh.

Section S3. EGOFET coupled with the microfluidic device
The transfer characteristics of the EGOFETs for both water ( Figure S1a) and glycine solution ( Figure S1b) display typical p-channel transistor responses, with a small current hysteresis and a high on/off ratio (10 3 -10 4 ).

Figure S2
shows that the slope of the line obtained when plotting (IDS) 0.5 vs VG (eq. 2) in the case of 3 M glycine decreases by 13.8% compared to water solution, while the intercept, which gives the threshold voltage (VTh), is shifted to positive voltage of around 70 mV.

. Electrical characterization of the EGOFET
A gold wire with a diameter of 0.5 mm was used as top gate, and was treated with 1H,1H,2H,2Hperfluorodecanethiol (PFDT) to achieve higher hydrophobicity ( Figure S3) in order to inhibit crystallization at the gate 3 . This was confirmed by eyes looking at the crystallization without Faraday cage, Figure S4 and movie S1: the crystals are appearing on the channel region.  The transfer and output curves obtained using water and a bare gold wire are shown in Figure S5 a and 5b, respectively. The transfer and output curves obtained using water and the PFDT functionalized gold wire are shown in Figure S5c and 5d, respectively. The transfer and output curves obtained using 1 M glycine solution and a bare gold wire are shown in Figure S5e and 5f, respectively. The transfer and output curves obtained using 1 M glycine solution and the PFDT functionalized gold wire are shown in Figure S5g

Section S4.3. Real-time measurements
All the experiments included in the main text are performed with a Faraday cage. However, we have also performed some measurements without the cage as this allows to visualize the crystals formation with a camera, by determining the induction time, in addition to the EGOFET recording.
The lack of the Faraday cage results in much higher noise level, hence quantitative measurements cannot be performed.
The electrical measurement and the optical image were recorded at the same time once the gold wire was inserted into the droplet (time = 0 s). A VG of -0.8 V and VDS of -0.7 V were applied.
First, a decrease in volume of the droplet due to evaporation was observed, as well as a decrease in the area of the contact with the gold wire ( Figure S4). The crystals were optically visible at the contact region of droplet with the semiconducting layer after 864 s, and fully covered the surface in just in 1.5 s. The induction corresponds to a sharp increase in the IDS (Figure S7), in accordance to previous results obtained with a microelectrode array 6 . Note that this peak in IDS is not observed when water is used as electrolyte and it is seen only when a glycine solution is used as electrolyte in all devices tested, Figure S8. The exact shape of the IDS peak may be different in each device, but the sharp increase in current at the induction time is seen in all devices.

Section S4.4. Stability of the EGOFET device over the crystallization process
To further verify the stability of the EGOFET device and exclude any effect from crystallization on the semiconducting layer, two experiments were carried out.
The first experiment was performed by measuring the conductivity of the semiconducting film by applying a bias voltage of 0.7 V between the source and drain contacts before adding a 3 M glycine droplet (before crystallization) and after washing out the crystals obtained from the crystallization (after crystallization). In between, the droplet was left to evaporate, leading to crystallization until no obvious color change can be observed. As shown in Figure S9, the current measured after crystallization recovers to the original value before crystallization. A slow increase of current was observed as a result of the stress measurement, which is commonly seen in organic semiconductors, while the trend is similar for the current curve before and after crystallization.
The second experiment was carried out by measuring the IDS of the EGOFET device by placing water droplet before and after adding a 3 M glycine droplet and leave it to crystallize. As shown in Figure S10, the IDS current recovers to the same value after water droplet was used again after removing the glycine crystals.
Therefore, based on the above experiments, it is clear that the EGOFET device is stable over the crystallization process, and there is no effect associated to the presence of the crystals on the semiconducting film.

Section S5. Calculations of the droplet crystallization dynamics Section S5.1. Critical concentration and supersaturation ratio
Under the classical nucleation theory, the concentration of molecules must be high enough to reach supersaturation to enable nucleation and crystal growth. In our previous work 6 , the critical concentration at which supersaturation is reached was determined from the time at which fluctuations in the recorded current were first observed by using an interdigitated electrode array.
In this study, due to the EGOFET sensitivity, this time can be evaluated more precisely and it can be assigned to the beginning of stage 1 (see main text). The critical concentration of glycine (Cgly*) can be calculated as: where C0, Vgly, Vevap are the initial concentration of glycine, the droplet volume, and the evaporated volume, respectively. As the glycine droplet volume is the same as that of the water droplet (VH2O) in our range of glycine concentrations, the above equation can be re-written as:  Table S1.

Section S5.2. Molecule concentrations and transfer rates
The amount of glycine molecules (Ni) in the EDL at a given time of ti can be calculated as: where Ci, Vi are the concentration of glycine molecules and volume of the EDL at ti, respectively.
Assuming the volume of EDL does not change significantly in Stage 1 and Stage 2 of crystallization (see main text), then the following eqs are valid: Where ∆C is the relative concentration change in each stage and C0 is the concentration at the beginning of each stage. Therefore, the transportation rate of glycine molecules in each stage, Ri, can be calculated as the ratio between the total change of the glycine number divided by the time associated to each stage: The change in concentration is found by measuring the changes in current and using eq. 3.
To give an example, in the case of device 1 (Figure 4a), we have determined Cgly* to be 4.7 M.
This is the concentration of glycine molecules at the beginning of stage 1 (i.e. at 1432.6 s for device 1, main text). The change in current is then correlated to the change in concentration (eq. 7), hence the concentration at the end of stage 1 (C1) can be calculated.  Table S2 reports all values used.
By using the same approach, we can also get the concentration at the end of stage 2 (C2). In this case, the current change is (141.3-96.2)/96.2 = 46.9%, which is larger than the change observed for glycine concentrations in the range 0.5 mM -3 M. Using eq. 3, we obtain C2 = 5x10 -6 M, which is well below the minimum concentration that we can detect with the EGOFET (Figure 2b).
Assuming that the concentration of molecules at the end of Stage 2 is so small to be approximated to zero, compared to the initial concentration, then a change in concentration of (0-6.0) M = -6.0 M is measured over Stage 2, which gives R2 = -6.0 M/14.9 s= -0.4 M/s. The above calculations are also applied to the other two devices. All values are summarized in Table S2.
Note that the water evaporation rate is calculated to be 2 µL/1821 s (S4.1), and the glycine concentration changes due to evaporation from stage 1 to 2 for device 1 (Figure 4a) can be calculated to be 0.016% and 0.8% respectively, which is negligible when compared to the concentration change (28.4% and 100% for stage 1 and 2 as shown in Table S3) due to crystallization. Thus, in all the calculations for the concentrations and transfer rates, the concentration changes ascribed to water evaporation are ignored.
20 Table S2. A summary of the characteristic times, IDS, relative current change (-∆I), and related molecules concentration (Ci) at the beginning and end of Stages 1 and 2 (extracted from Figure 3 and Figure S8).  Table S3. Quantitative analysis of the crystallization dynamics. Table reporting the relative IDS change (-∆I/I0), the relative concentration change (∆C/C0) from the beginning to the end of each stage and the corresponding molecular transport rate (R) calculated for each stage, as extracted from Figure 3 and Figure S8. Measurements have been performed on 3 different devices.