📄 chemistry

Dual-Mode Colorimetric and RGB Detection of Sulfide Based on a Magnetic Fe 3 O 4 /CoFe 2 O 4 /Ce-BDC Hybrid Nanozyme

This study develops a dual-mode colorimetric and RGB sensing platform using a magnetic Fe3O4/CoFe2O4/Ce-BDC nanozyme that detects sulfide via ROS-mediated reduction of oxidized TMB, enabling rapid, portable, and quantitative analysis in environmental and biological samples through both spectrophotometry and smartphone-based imaging.

Original authors: Qingyuan Weng, Xinyi Wen, Tianyuan Wei, Qing Zhang, Wanying Zhu, Junli Hong

Published 2026-07-01

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Qingyuan Weng, Xinyi Wen, Tianyuan Wei, Qing Zhang, Wanying Zhu, Junli Hong

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A "Magic" Detective for Poisonous Gas

Imagine you are trying to find a tiny, invisible, and dangerous intruder (sulfide gas) in a large room. This intruder is bad news for both nature and human health, but it's hard to spot without expensive, heavy machinery.

This paper introduces a new, clever "detective" team made of tiny magnetic particles. This team can sniff out the intruder and change color to show you exactly how much is there. The best part? You can read the results either with a fancy lab machine or just by taking a picture with your smartphone.

The Detective Team: The "Nanozyme"

The core of this invention is a special material called FCF/Ce-BDC. Think of this as a tiny, three-part superhero team:

The Magnet (Fe₃O₄): This part acts like a magnetic anchor. It lets scientists easily grab the detective particles with a magnet and pull them out of the water so they can be used again and again.
The Engine (CoFe₂O₄): This part is the power source. It helps the team work faster and harder.
The Sponge (Ce-BDC): This is a porous, sponge-like structure that catches the intruder (sulfide) and brings it right to the team's face.

Together, these three parts create a "Nanozyme." A nanozyme is a tiny machine that acts like a natural enzyme (a biological catalyst) but is made of metal. Its job is to start a chemical reaction that creates a bright blue color.

How the Detection Works: The "Blue Light" Switch

The system uses a simple chemical trick involving two ingredients: TMB (a clear liquid) and Hydrogen Peroxide (like the stuff in first-aid kits).

The "ON" Switch: When the Nanozyme detective team meets the TMB and Peroxide, it acts like a spark plug. It turns the clear liquid into a deep blue liquid. This is the "normal" state.
The "OFF" Switch: If Sulfide (the intruder) is present, it acts like a fire extinguisher. It rushes in and:
- Steals the "fuel" (reactive oxygen species) the Nanozyme needs to make the blue color.
- Directly turns the blue color back into clear liquid.
- Blocks the Nanozyme's face so it can't work.

The Result: The more sulfide there is, the less blue color you see. The liquid fades from deep blue to colorless. It's like a dimmer switch: more poison = less light.

Two Ways to Read the Results

The paper describes two ways to measure this color change, making it useful for both scientists and regular people in the field:

The Lab Mode (Spectrophotometer): Scientists use a machine to measure exactly how much light the blue liquid absorbs. This is super precise and can detect very tiny amounts (as low as 0.29 micromoles).
The Smartphone Mode (RGB): You can just take a photo of the liquid with your phone. An app analyzes the "Red, Green, and Blue" pixels in the photo. If the blue pixels are missing, the app knows sulfide is there. This is slightly less sensitive (detecting down to 31.26 micromoles) but perfect for quick checks outside the lab.

Portable Tools: The "Swab, Strip, and Gel"

To make this easy to use in the real world (like in a lake or a hospital), the researchers built three different "carriers" to hold the Nanozyme:

The Hydrogel Film: Think of this as a jelly sandwich. The Nanozyme is trapped inside a jelly-like block. This controls the reaction speed perfectly, making the color change very consistent.
The Cotton Fabric Strip: This works like a paper towel. When you dip it in water, the liquid soaks up the cotton fibers, which naturally concentrate the sulfide. This makes the test more sensitive and helps filter out dirt that might confuse the result.
The Cotton Swab: This is like a magic Q-tip. You dip the tip in the sample, then break the stick to let the chemicals inside the swab flow down to the tip. The chemicals mix themselves right there on the tip, creating a color change in just a few minutes.

Did It Work?

The researchers tested this system in two tricky environments:

Lake Water: It worked great, detecting sulfide accurately even with all the natural junk in the water.
Human Blood: They tested it on human serum (the liquid part of blood). It worked well here too, though the blood's natural chemistry made it slightly harder to get a perfect reading.

They also checked if other things (like salt or other metals) would trick the sensor. The answer was no. The sensor is very specific to sulfide and ignores the "noise" from other common chemicals.

Summary

This paper presents a new, low-cost way to find dangerous sulfide gas. By combining a magnetic, sponge-like nanozyme with simple color-changing chemistry, they created a system that can be read by a lab machine or a smartphone. They packaged it into easy-to-use tools like cotton swabs and fabric strips, making it possible to check for this poison quickly in lakes or even in human blood samples without needing a massive laboratory.

Problem Statement

Sulfide ( $S^{2-}$ ) and hydrogen sulfide are ubiquitous, highly reactive, and toxic anions found in natural waters, industrial wastewater, and food processing chains. Even at trace levels, they pose severe risks to aquatic ecosystems and human health, potentially causing respiratory failure, neurological disorders, and tissue hypoxia. While conventional analytical methods (e.g., electrochemical sensors, chromatography, and spectroscopy) offer high sensitivity, they are often limited by high costs, complex sample preparation, and the need for specialized operators, making them unsuitable for rapid, decentralized field screening. There is a critical need for rapid, low-cost, and user-friendly detection methods that can operate in complex environmental and biological matrices without compromising analytical reliability.

Methodology

The study proposes a dual-mode sensing strategy integrating a novel magnetic nanozyme with a TMB/ $H_2O_2$ chromogenic system.

Nanozyme Synthesis: The authors synthesized a ternary hybrid nanozyme, denoted as FCF/Ce-BDC, composed of magnetic $Fe_3O_4$ , spinel $CoFe_2O_4$ , and a Ce-based metal-organic framework (Ce-BDC). This composite was designed to possess peroxidase (POD)-like activity, magnetic recoverability, and a hierarchical porous structure.
Detection Mechanism:
- Catalysis: In an acidic environment (pH 3.5), the FCF/Ce-BDC nanozyme catalyzes the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by hydrogen peroxide ( $H_2O_2$ ), generating a blue oxidized product (oxTMB) with a maximum absorbance at 651 nm.
- Inhibition/Reduction: The presence of sulfide interferes with this reaction through three complementary pathways: (1) scavenging reactive oxygen species (ROS), particularly hydroxyl radicals ( $\cdot OH$ ); (2) chemically reducing the blue oxTMB back to its colorless form; and (3) partially inhibiting the nanozyme's catalytic activity by interacting with surface metal sites.
- Signal Output: This interference results in a concentration-dependent decrease in absorbance at 651 nm and a visible color transition from blue to colorless.
Dual-Mode Readout:
- Spectrophotometric Mode: Quantitative analysis via UV-Vis absorbance measurement.
- RGB Mode: Quantitative analysis via smartphone-based imaging and RGB color extraction (specifically the G-B channel metric), enabling field-deployable detection.
Sensor Fabrication: To translate the chemistry into portable formats, the researchers immobilized the nanozyme and reagents on three distinct platforms:
- Hydrogel Film: Utilizes ion-mediated crosslinking to modulate reaction kinetics.
- Cotton Fabric Strips: Exploits capillary-driven fluid transport for analyte enrichment.
- Cotton Swabs: Features a self-mixing mechanism where breaking the swab handle drives reagent flow to the tip for rapid reaction.

Key Results

Material Characterization: The FCF/Ce-BDC composite exhibited a hierarchical mesoporous structure (specific surface area: 17.0077 $m^2 \cdot g^{-1}$ ; pore diameter: ~23.26 nm) and strong magnetic responsiveness, allowing for separation within 10 seconds. XPS and elemental mapping confirmed the uniform distribution of Fe, Co, Ce, O, and C.
Catalytic Performance: Kinetic studies revealed that the ternary composite possessed superior POD-like activity compared to bare $Fe_3O_4$ . The synergistic interaction between Fe, Co, and Ce centers facilitated efficient $H_2O_2$ activation and ROS generation.
Detection Limits:
- Absorbance Mode: Achieved a limit of detection (LOD) of 0.29 µM.
- RGB Mode: Achieved an LOD of 31.26 µM.
- The absorbance mode demonstrated a lower detection limit than the standard HJ 1226–2021 methylene blue spectrophotometric method (LOD = 1.03 µM).
Sensor Performance:
- The hydrogel sensor achieved an LOD of 28.72 µM.
- The cotton swab sensor achieved an LOD of 29.74 µM.
- The cotton fabric sensor achieved an LOD of 30.58 µM.
- All portable sensors showed linear responses in the 10–300 µM range with high correlation coefficients ( $R^2 > 0.97$ ).
Selectivity and Stability: The system showed high specificity for sulfide against common interfering ions (e.g., $Pb^{2+}$ , $Ca^{2+}$ , $SO_4^{2-}$ ). The nanozyme retained catalytic performance after 120 days of storage at 4°C.
Real Sample Analysis: The method was validated in spiked human serum and lake water. Recovery rates ranged from 95.5% to 99.9% in serum and 96.11% to 104.68% in lake water across both spectrophotometric and RGB modes.

Significance and Claims

The authors claim that this work advances the rational design of multifunctional nanozymes by integrating magnetic recoverability, high catalytic efficiency, and a hierarchical porous structure. The primary significance lies in the delivery of a validated, application-ready framework for sulfide monitoring that bridges the gap between laboratory-grade accuracy and field-deployable convenience.

Key contributions highlighted by the authors include:

Dual-Mode Flexibility: The ability to perform quantitative analysis via both precise spectrophotometry and accessible smartphone RGB imaging allows for versatile use in both centralized labs and remote field settings.
Portable Platform Design: The development of hydrogel, cotton fabric, and cotton swab sensors provides a modular strategy for on-site detection that requires minimal user training and no dedicated analytical equipment.
Practical Utility: The system addresses major constraints in environmental and human health monitoring by offering a cost-effective, rapid (reaction times of 5–20 minutes), and operationally simple method for tracking target pollutants in complex matrices like whole blood and environmental water.

The paper concludes that this integrated approach offers a reliable tool for routine environmental monitoring and rapid sulfide screening in health-related contexts, capable of providing real-time hazard warnings without compromising analytical reliability.