Grantee Research Project Results

Final Report: Scalable 2D semiconductor-based field-effect transistors for rapid and efficient detection of lead ions

EPA Grant Number: SU840577
Title: Scalable 2D semiconductor-based field-effect transistors for rapid and efficient detection of lead ions
Investigators: Zhao, Mark , Zhang, Wen , Alam-Sabuj, Md Mohidul , Naseer, Mariam
Institution: New Jersey Institute of Technology
EPA Project Officer: Spatz, Kyle
Phase: I
Project Period: August 1, 2023 through July 31, 2024
Project Amount: $24,999
RFA: 19th Annual P3 Awards: A National Student Design Competition Focusing on People, Prosperity and the Planet Request for Applications (RFA) (2022) RFA Text |  Recipients Lists
Research Category: P3 Awards , P3 Challenge Area - Safe and Sustainable Water Resources

Objective:

It is repeatedly reported that many U.S. residents, epically school children, are still exposed to lead in drinking water that exceeds the EPA's Action Level, which could result in adverse health effects over a lifetime of exposure. Onsite monitoring lead in drinking water would reap billions of dollars in human health and ecological security. In this EPA P3 phase I project, we aimed to develop scalable two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) based field-effect transistor (FET) sensors for rapid and in-situ lead ion detection in drinking water. Our central hypothesis was that 2D semiconducting TMDs can be used as the channel materials for the low-cost, large-scale fabrication of Pb²⁺ FET sensors with high sensitivity and selectivity. To address the challenge in TMD degradation, we further hypothesize the use of  lab-grown monoalyer hexagonal boron nitride (hBN) films (~1 nm thick) to encapsulate and stabilize the TMD channel materials. To address the challenge if efficient functionalization of TMD channel materials with typical Pb²⁺ probe molecule, L-glutathione (L-GSH), we hypothesized a universal functionalization process based on the combination of hBN intermediate layer and pyrene-based linker molecule.

Objectives of the research include 1) fabrication of the TMD-based FET sensors using large-area, uniform monolayer TMDs; 2) systematical investigation of the sensors' sensitivity, selectivity, stability, and reusability for both synthetic and real-life tap water. The simple device design contributes to the cost-effective fabrication and potential integration for portable and in-home uses. Besides, synergistic educational activities such as new teaching materials and workshops will be operated to involve undergraduates and graduates in different STEM disciplines to learn about the EPA P3 principles and concepts and importance of novel sensing technologies for safeguarding human health from water pollutants.

Summary/Accomplishments (Outputs/Outcomes):

1. Fabrication of TMD-based FET sensors for Pb²⁺ ion detection

As mentioned in the project proposal, we have pre-designed a FET array that has 10 set of FET electrodes with 10 FET devices in each set in an area of 1 x  2 inch². In the current project, we have successfully fabricated 100 2D MoS₂ and WS₂-based FET devices on each 1 x  2 inch² Si chip using this FET arrays. The high-density 2D MoS₂ and WS₂ flakes were synthesized in our lab through our well-developed NaCl-assisted, spin-coating based chemical vapor deposition (CVD) process. Large-area 2D MoS₂ and WS₂-based FET devices were fabricated after transferring the as-grown MoS₂ and WS₂ flakes on the FET channel areas (Figure 1). Tests of these devices showed that more than 95% of the 2D MoS₂-based FET devices perform well. However, the performance of WS₂-based FET devices was not as good as expected due to the transfer issues. As a result, in this project, we focused on the fabrication of 2D MoS₂-based FET sensors for Pb²⁺ ion detection.  

After the fabrication of 2D MoS₂-based FET devices, a lab-grown hBN film was transferred on the top of MoS₂ channel area to encapsulate the stabilize the MoS₂ channel material. The results indicated that the performance of 2D MoS₂-based FET devices was improved after the encapsulation of hBN films, which was stable for more than 3 months when exposed in air at room temperature.

 

Figure 1. (a) Digital images showing the as-fabricated 2D MoS₂-based FET Pb²⁺ sensors; (b, c) Zoom-in image showing the spanning of MoS₂ and the hBN film over the channel area.

The 2D MoS₂-based FET Pb²⁺ sensors were fabricated after functionalizing the 2D MoS₂ with the typical Pb²⁺ probe molecule, L-GSH. This functionalization was conducted using our recently developed hBN-enabled functionalization process assisted by a Pyrenebutyric acid N-hydroxysuccinimide ester (pyrene-NHS, PBASE) linker molecule. During the process, the pyrene group in the linker molecule bonded with the hBN film through strong π - π  interaction, while the NHS group in the linker molecule bonded to the amino group in L-GSH, leading to the formation of 2D MoS₂-based FET sensors that can selectively detection Pb²⁺ ions.      

2. Determine the response time, sensitivity, and selectivity of 2D MoS₂-based FET Pb²⁺ sensors

We assessed the sensing performance of our as-fabricated 2D MoS₂-based FET Pb²⁺ sensors using the lab-built probe station equipped with a probe card. The Pb²⁺ detection was performed using synthetic solutions first by dissolving lead nitrate in DI water at different concentrations (0.01ppn to 10 ppm). We systematically investigated the response time, sensitivity, and selectivity of the FET sensors for Pb²⁺ detection.

The response time, 𝜏 , was determined by tracking the changes of source-drain current, I_ds ( Δ I_ds), vs. time after ping Pb²⁺ solution. It was noticed that the value of 𝜏  varied with the concentration of Pb²⁺ ion. A short response time 𝜏  of 3 seconds was confirmed when detecting the 0.01 to 1 ppb Pb²⁺ ion solution, which extended to 6 seconds for the 10 ppm Pb²⁺ ion solution.

To determine the sensitivity (limit of detection, LoD), the Δ I_ds before and after the casting of Pb²⁺ solutions will be recorded at fixed gate (V_g) and source-drain (V_ds) voltages. Obvious change of I_ds was observed even when the detection of 0.01 ppb Pb²⁺ ion solution (Figure 2a). Linear calibration curves of Δ I_ds/I₀ vs. logC(Pb²⁺) were obtained for all different gate voltages (10, 15, and 20V), where I₀ is the source-drain current before Pb²⁺ detection and C(Pb²⁺) is the concentration of Pb²⁺ in the solution (Figure 2b). Fitting of the calibration curve at 20V gives the equation: Δ I_ds/I₀ = 0.0812 logC(Pb²⁺) + 0.346. The LoD values calculated from the testing results were around 0.07 ppb, much lower than the EPA Action Level of 15 ppb and new Trigger Level of 10 ppb.

 

Figure 2. (a) ) I_ds-V_g profiles of the 2D MoS₂-based FET sensor during the fabrication and detection of 0.01 ppb Pb²⁺ ion in DI water; (b) Calibration curves of the FET sensor at three different gate voltages at 10V, 15V and 20V.

The selectivity to Pb²⁺ ion detection of the 2D MoS₂-based FET sensors was determined by testing solutions of Pb²⁺, Na⁺, Mg²⁺, Zn²⁺, Cd²⁺, Cu²⁺, and their mixture with varied Pb²⁺ concentrations and compare the sensing response Δ I_ds/I₀. The results showed that the as-fabricated FET sensors exhibited negligible response to Na⁺, Mg²⁺, Zn²⁺, and Cd²⁺ ions, while the Cu2+ is considered as a major interference ions. However, the selectivity to Pb²⁺ vs Cu²⁺ still showed a high value of ~85%, which was in the range of 93-98% for the other 4 ions. The sensors also exhibited a good selectivity to Pb²⁺ even when mixing all these different ions together.

In addition to the DI water-based synthetic solutions, we also assessed the Pb²⁺ sensing performance of the as-fabricated FET sensors in tap water, which contains large amount of Al³⁺. Fe³⁺, Ni²⁺, Cu²⁺, and Zn²⁺, but with negligible Pb²⁺. Therefore, we added different concentration of Pb²⁺ ions in the tap water for the sensing tests. Very similar calibration curves to those obtained for DI water-based solutions were obtained. This further demonstrated the good selectivity of our FET sensors to Pb²⁺ detection, and also indicated the applicable of our FET sensors for Pb²⁺ detection in field tap water.  

3. Evaluate the stability and reusability of 2D MoS₂-based FET Pb²⁺ sensors

The stability and reusability are two other important characteristics to evaluate the performance of environmental chemical sensors. Here, the stability was evaluated by exposing the sensors in air at room temperature for varied durations (from several days to one months). No obvious sensing response changes were observed even after one month of exposure, indicating the good stability of our fabricated FET sensors.

To evaluate the reusability of the FET sensors for Pb²⁺ detection, we used 100 mM oxalic acid to regenerate the 2D MoS₂-based FET sensors by washing the bound Pb²⁺ ions away. The regenerated sensors were used to detection Pb2+ again, and very similar sensing responses were observed, indicating a good reusability of our fabricated FET sensors.

4. Involvements of state agencies and students in research and education activities

Some of the phase I research results were shared with the Meadowlands Research and Restoration Institute (MRRI) of New Jersey Sports and Exposition Authority (NJSEA), who provided cross check of the analytical results of Pb²⁺ in water. The PI has integrated the related research results in his courses in the Materials Engineering curriculum MTEN 201 Introduction of Materials Principles and MTEN 610 Fundamentals of Materials Science. The Co-PI has developed and integrated some new course modules related to the Pb²⁺ detection in water in the environmental engineering curriculum (ENE 262): Introduction to Environmental Engineering that is taught to sophomore and junior civil engineering students. 

In addition, the PI has incorporated the related research results into his 2024 one-week summer program at NJIT for high-school students to perform hands on materials science activities. The PI also introduced the research results to NJIT undergraduate students in materials engineering through the two-day Materials Engineering workshop.

Conclusions:

This EPA P3 phase I research has successfully developed 2D semiconducting MoS₂-based FET sensors for Pb²⁺ detection in both synthetic and tap water. These FET sensors can give reliable sensing results within 3-6 seconds. Linear calibration curves with an equation of Response ( Δ I_ds/I₀) = 0.0812 logC(Pb²⁺) + 0.346 were obtained, which can be used as a standard for the Pb²⁺ detection in water. The as-fabricated FET sensors also showed good selectivity to Pb²⁺ detection compared with various inorganic interference ions. Furthermore, our 2D MoS₂-based Pb²⁺ FET sensors were demonstrated to be with good stability upon exposing in air for a couple of months. They also showed good reusability after the sensor regeneration by washing with oxalic acid. In general, our research team has successfully demonstrated the proof-of-concept 2D MoS₂-based FET sensors for rapid, sensitive, and selective detection of Pb²⁺ ions in water and carried out some preliminary field tests, which represents a potentially game-changing detection technology that could improve the monitoring efficiency of lead exposure in drinking water systems to safeguard human health.  

Several challenges still exist for the further development of such Pb²⁺ FET sensors. First, these FET sensors were fabricated through typical photolithography process conducted in cleanroom environment, which could potentially result in a high fabrication cost. Second, the current tests of these FET sensors were conducted in a lab-built probe station equipped with Keithley sourcemeter and electrometers and a switch matrix for signal transfer and processing, which is not deployable for on-site monitoring of Pb²⁺ ions. To address these challenges, in the EPA P3 Phase II research, there are pressing needs to optimize the sensor fabrication process to reduce the fabrication costs and to integrate the sensor testing system into a portable device for the development of prototype 2D MoS₂-based Pb²⁺ FET sensors for real-time, on-site monitoring of Pb²⁺ ions in water.  

Journal Articles:

No journal articles submitted with this report: View all 3 publications for this project

Supplemental Keywords:

field-effect transistor sensor, Pb²⁺ ion detection, 2D semiconductors, response time, sensitivity, selectivity, stability, portable device, prototype, real-time monitoring, field tests