The article focuses on a major environmental and technological problem we face today: the contamination of soil and water by harmful chemicals. Among these chemicals, one of the most dangerous is 4-nitrophenol (4-NP), a toxic compound commonly used in industries such as dyes, pesticides, pharmaceuticals, and chemical manufacturing. Because of its severe effects on human health and the environment, detecting even very small amounts of 4-NP in water is extremely important.

The study presents a new electrochemical sensor made from chemically exfoliated MXene (Ti₃C₂Tₓ) and shows that this sensor can detect very low concentrations of 4-nitrophenol quickly, accurately, and with high sensitivity. MXenes are a new class of two-dimensional (2D) materials with remarkable electrical, chemical, and mechanical properties. In this research, MXene was used to create sensor electrodes capable of identifying tiny amounts of 4-NP in drinking water.

Below is a detailed, yet simple and easy-to-follow explanation of the entire study, the scientific background, how the MXene was made, how the sensor works, how it was tested, and why it is important.

1. Environmental Background and Why This Research Matters

The purity of soil and water directly affects agriculture, food safety, plant growth, and human health. Over the last century, industrialization, rapid urban growth, and the intensive use of chemicals like fertilizers and pesticides have greatly increased pollution. Many of these pollutants stay in the environment for long periods and continue to harm living organisms.

Among the many contaminants, nitrophenols—especially 4-nitrophenol—are a major concern. They come from industrial wastewaters, manufacturing activities, dyes, leather processing, pharmaceuticals, and pesticides. These compounds are toxic because they disrupt biological processes, cause genetic damage, and can harm major organs like the liver and kidneys. They are also carcinogenic.

Because 4-NP is harmful even at very tiny concentrations, the U.S. Environmental Protection Agency (EPA) has set a strict limit: drinking water should contain less than 0.43 µM of 4-NP. Exceeding this amount can cause health problems including:

• methemoglobinemia

• damage to the liver and kidneys

• nausea

• lethargy

• cyanosis

• disruption of metabolism

Thus, it is essential to monitor water quality continuously to identify 4-NP even at very low levels.

Traditional methods for detecting chemicals include:

• high-performance liquid chromatography

• spectroscopic methods

• fluorescence analysis

• electrophoresis

• chemiluminescence

While effective, these methods are costly, require trained personnel, and are not suitable for rapid or on-site detection.

This is where electrochemical sensors come in. They offer:

• rapid response

• low cost

• portability

• real-time monitoring

• high levels of sensitivity

• no need for complex lab infrastructure

Research in electrochemical sensors has accelerated dramatically thanks to the discovery of 2D materials like graphene, MoS₂, and especially MXenes.

2. Why MXenes Are Exciting Materials for Sensors

MXenes (with the structure Mn₊₁XₙTₓ) are a family of two-dimensional materials made by chemically etching a layer out of a 3D material called MAX phase. In this study, the MAX phase Ti₃AlC₂ was transformed into Ti₃C₂Tₓ, a MXene.

MXenes have gained attention because of their impressive properties:

Major Advantages of MXenes

1. Metal-like electrical conductivity—approximately 6000–8000 S/cm, extremely high for 2D materials

2. Hydrophilicity—they interact well with water, making them easy to process

3. Large surface area—ideal for sensing applications

4. Surface functionalization—their surfaces naturally host groups like –OH, –F, and =O

5. Fast electron transfer—essential for fast sensor responses

6. Chemical stability

7. Compatibility with biological and environmental systems

Because MXenes combine high conductivity with active surface chemistry, they are excellent candidates for environmental sensors that rely on charge transfer and molecular interactions.

Why Ti₃C₂Tₓ MXene Is Special

Ti₃C₂Tₓ was the first MXene discovered and remains the most extensively studied. It has been used previously to develop sensors for:

• glucose

• pesticides

• pharmaceutical compounds

• phenolic contaminants

However, earlier MXene-based 4-NP sensors had limitations such as lower sensitivity or poor selectivity.

3. Goal of the Study

The main objective of this research was to develop a highly sensitive, selective, and reliable MXene-based electrochemical sensor specifically for detecting extremely small quantities of 4-nitrophenol.

The authors wanted to achieve:

• higher sensitivity than previous sensors

• lower detection limits (ideally below EPA standards)

• excellent selectivity, even when other chemicals are present

• good reproducibility and stability

• practical usability by testing in real tap water

Importantly, they created the sensor using pure MXene without adding nanoparticles, enzymes, or composite materials. This shows the natural capability of MXene itself as a powerful sensing material.

4. MXene Synthesis Process (Simplified Explanation)

The researchers began by producing Ti₃C₂Tₓ MXene from its parent MAX phase, Ti₃AlC₂. The process involves selectively removing the aluminum (Al) layer using hydrofluoric acid (HF).

Basic Steps:

1. Mixing MAX powder with HF

   • Done very carefully because the reaction is exothermic

   • Occurs at 50°C for approximately 36 hours with constant stirring

2. Etching away the Al layers

   • This transforms the 3D MAX phase into a 2D MXene structure

   • The result is a stack of thin MXene sheets

3. Repeated washing and centrifugation

   • Removes acids and reaction by-products

   • Washing continues until pH reaches 5–6

4. Vacuum filtration and drying

   • The final MXene “cake” is dried and stored to avoid oxidation

This chemical exfoliation step is critical because high-quality exfoliation leads to better sensor performance.

5. Structural and Chemical Characterization of MXene

The study used various analytical techniques to confirm successful synthesis and to examine the structure of the MXene.

a. SEM Imaging

• Shows multilayered, accordion-like stacked MXene sheets

• The structure becomes more expanded compared to the compact MAX phase

• Thick but layered plates with exposed edges, useful for sensing

b. TEM and HR-TEM

• Confirms clear layered architecture

• Shows nanoscale edge exposure

• d-spacing measured around 0.817 nm, confirming layer separation

c. EDS (Energy Dispersive Spectroscopy)

• Detects titanium (Ti) and carbon (C) as expected

• Small amount of remaining aluminum (Al) from the MAX phase

• Presence of oxygen and fluorine from surface functional groups

d. FTIR

• Shows vibrations corresponding to Ti–C and O–H functional groups

e. XRD

• Confirms removal of aluminum

• Shows the expected shift in peaks that indicate layer expansion

f. XPS

• Identifies the oxidation state of Ti

• Surface chemical groups (–OH, –F, =O) are clearly observed

All analysis confirms the successful formation of high-quality Ti₃C₂Tₓ MXene suitable for sensing applications.

6. Development of the Electrochemical Sensor

The researchers prepared MXene-coated glassy carbon electrodes (MXene/GCE):

1. MXene powder was mixed into ethanol and sonicated to create a uniform slurry.

2. A few microliters of this mixture were dropped on a cleaned GCE surface.

3. The coated electrode was dried and rinsed.

This created a thin layer of stacked MXene on the electrode, forming the sensing surface.

7. How the Sensor Detects 4-Nitrophenol

MXene’s exceptional conductivity and surface chemistry enable it to interact strongly with 4-NP molecules.

Mechanism (simplified):

• MXene acts as a platform for rapid electron transfer.

• 4-NP undergoes electrochemical reduction on the MXene surface.

• This reduction produces a measurable electrical signal.

• The peak current is directly proportional to the concentration of 4-NP.

Because MXene has abundant electron-rich sites and active surface groups, the interaction and electron transfer between MXene and 4-NP occur efficiently.

8. Electrochemical Performance

Cyclic Voltammetry (CV) Results

• The current increases as scan rate increases, confirming active charge transfer.

• MXene/GCE shows much higher current response than bare GCE.

• Charge transfer resistance decreases significantly (249 Ω vs. 527 Ω).

Electrochemical Impedance Spectroscopy (EIS)

• Confirms faster electron transfer when MXene is present.

• Indicates improved electrical conductivity of the electrode.

Overall Findings

MXene enables fast, efficient, and highly sensitive detection of 4-NP.

9. Sensitivity and Limit of Detection (LOD)

Using differential pulse voltammetry (DPV):

• Wide detection range:
  500 nM to 100 μM

• High sensitivity:
  16.35 μA μM⁻¹ cm⁻²

• Very low detection limit:
  42 nM (0.042 μM)

This LOD is well below the EPA drinking water limit of 0.43 μM.
Thus, the MXene sensor is suitable for real environmental monitoring.

10. Selectivity of the Sensor

A sensor must distinguish the target chemical from others. The researchers tested several interfering substances:

• chlorophenol

• aminophenol

• phenol

• benzoic acid

• sodium, potassium, magnesium ions

Even with high concentrations of these chemicals, the sensor still detected 4-NP clearly and accurately.

Why the sensor is selective:

• MXene has surface functional groups (–OH, –F, =O) that interact specifically with 4-NP.

• The nitro group in 4-NP undergoes an electrochemical reduction more easily than similar molecules.

Interference effect was less than 6%, showing excellent selectivity.

11. Reproducibility, Stability, and Real-World Testing

Reproducibility

• Multiple MXene/GCE electrodes were prepared.

• All showed very similar results.

• Standard deviation was only 1.44%, indicating excellent reproducibility.

Stability

• After multiple electrochemical cycles, MXene sheets remained intact.

• SEM and EDS showed no major structural damage.

Testing with Real Tap Water

The final and most important step was testing the sensor with actual tap water (acidified to pH 3 to match optimal conditions). The results were highly accurate:

• Recovery between 95% and 99%

• Low standard deviation (1.6–3.3%)

This proves the sensor can work effectively in real environmental samples.

12. Overall Conclusions

The study successfully developed a high-performance Ti₃C₂Tₓ MXene-based electrochemical sensor for detecting 4-nitrophenol in water.

Key Achievements:

• Very high sensitivity

• Low detection limits far below EPA standards

• Broad linear detection range

• Excellent selectivity even in the presence of other chemicals

• Good stability and reproducibility

• Successful real-water testing

• Simple fabrication without additional nanoparticles, binders, or enzymes

The study highlights the potential of MXenes as powerful materials for environmental monitoring technologies. Because of their inherent conductivity, tunable surface chemistry, and layered structure, MXenes can be further developed for sensing many toxic compounds, not just 4-NP.