3.1.2. Alcohol-Oxidase-Based Formaldehyde Sensors
Alcohol oxidase (EC 1.1.3.13) is a crucial enzyme that catalyzes the oxidation of primary alcohols, including methanol. Studies have revealed that AOX can also catalyze the oxidation of formaldehyde to produce formic acid. AOX itself undergoes reduction as it accepts electrons from both methanol and formaldehyde. The reduced form of AOX is subsequently regenerated through oxidation by molecular oxygen (O2
), leading to the generation of hydrogen peroxide (H2
O2) as a byproduct. The overall reaction can be summarized as follows:
) as a byproduct. The overall reaction can be summarized as follows:
Therefore, AOX can be employed to fabricate electrochemical sensors for formaldehyde detection. Depending on the products and processes involved in the catalytic reaction, various types of electrochemical sensors can be developed. For instance, since formic acid can release hydrogen ions, a potential-based sensor with ion-selective membranes for hydrogen ion recognition can be constructed [78,81].
3.2. Electrochemical Sensors Rely on Electrocatalysts
Therefore, AOX can be employed to fabricate electrochemical sensors for formaldehyde detection. Depending on the products and processes involved in the catalytic reaction, various types of electrochemical sensors can be developed. For instance, since formic acid can release hydrogen ions, a potential-based sensor with ion-selective membranes for hydrogen ion recognition can be constructed [49][50].
3.2. Electrochemical Sensors Rely on Electrocatalysts
3.2.1. Noble-Metal-Based Formaldehyde Sensors
Noble metal catalysts are important electrocatalysts, widely utilized in various fields such as oxygen reduction, hydrogen evolution, oxygen evolution, CO
2 electroreduction, and oxidation of organic small molecules. Precious metal catalysts, including gold (Au), platinum (Pt), palladium (Pd), and silver (Ag), have also proven highly effective in the electrocatalytic oxidation of formaldehyde. They can efficiently and directly convert formaldehyde into carbon dioxide and water through different pathways, making them essential materials for constructing electrochemical formaldehyde sensors.
Au-based electrocatalysts
In as early as the year 2000, Hauser et al. devised a method for the direct amperometric detection of low concentrations of formaldehyde in the gas phase
[51][84]. This method involved depositing gold onto a Nafion membrane as the working electrode. The study also explored the impact of gas flow rate and gas stream humidity on the sensor’s performance, along with its sensitivity to various organic and inorganic gases. Furthermore, the researchers proposed a solution to counter interference from NO, NO
2, and SO
2 by selectively adsorbing formaldehyde from the sample stream using an aluminum oxide filter. This approach boasts a low detection limit and an extensive dynamic range, making it highly suitable for continuous air monitoring applications. Subsequently, Surareungchai et al. achieved the sensing of formaldehyde using an unmodified gold electrode through the application of pulsed amperometric detection (PAD) in a flow injection (FI) system
[52][85].
In recent years, there have been studies employing various methods to prepare nanostructured gold materials with precise structural features, aiming to enhance their catalytic performance. Kumar et al. utilized an in situ electrochemical process to fabricate gold (111)-oriented nanoparticles on the surface of carbon nanofibers–chitosan composite (GCE/CNF-CHIT@Au
nano)
[53][87]. This innovative approach resulted in the development of a sensor capable of detecting formaldehyde in buffered solutions. Through a range of physicochemical characterization techniques, it was revealed that amino-functionalized chitosan stabilizes gold (111) nanoparticles with a size of approximately 10 nm within the composite matrix. In comparison to other catalysts, these Au(111) catalysts exhibit exceptional catalytic activity, stability, and remarkable surface characteristics. The mechanism of formaldehyde electrocatalytic oxidation on the GCE/CNF-CHIT@Au
nano modified electrode was investigated using cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM). During the anodic scan, formaldehyde exhibited oxidation onset at 0.15 V, followed by a broad anodic peak at 0.4 V. In the reverse scan, a well-defined anodic peak was observed at the same potential. The mechanistic pathway for formaldehyde oxidation involves two possible routes: a direct pathway (dehydrogenation) and an indirect pathway (dehydration) via the adsorption of CO as a poisoning species.
Deposition of gold nanoparticles onto polymer surfaces has proven to be an effective technique, allowing for the efficient binding and dispersion of nanoparticles due to the functional groups present on the polymer surface. Huang et al. were the first to modify a glassy carbon electrode (GCE) with polypyrrole (PPy) and subsequently electrodeposit gold nanoparticles (AuNPs), resulting in a functional electrode denoted as GCE/PPy/AuNPs
[54][88]. This electrode exhibited high electrical conductivity and excellent catalytic activity, enabling the sensitive quantification of formaldehyde. Liu et al. presented the development of a molecularly imprinted electrochemical sensing platform tailored for the specific detection of formaldehyde
[55][89]. They immobilized gold nanoclusters (Au NCs) onto the surface of polydopamine nanospheres (pDA NPs) to create a pDA@Au NCs composite material. Subsequently, molecularly imprinted polymers (MIPs) for formaldehyde were synthesized on the surface of pDA@Au NCs, yielding the pDA@Au NCs-MIPs-HOPG (highly oriented pyrolytic graphite) electrode. This sensor, combining molecular imprinting technology (MIT) with the catalytic properties of noble metal nanoparticles (Au NCs), demonstrated impressive selectivity and high sensitivity. It featured a broad detection range spanning from 0.2 μM to 0.02 M and a low detection limit of 0.1 μM. The sensor was effectively applied to detect trace amounts of formaldehyde residues in seafood, particularly octopuses, showcasing satisfactory selectivity and reproducibility.
Pt-based electrocatalysts
Similar to gold nanoparticles, platinum nanoparticles also possess high specific surface area, excellent electrocatalytic activity, and stability. They have significant potential for use as electrocatalysts in constructing formaldehyde electrochemical sensors. Platinum nanoparticles can promote the oxidation reaction of formaldehyde, thereby enhancing the current response and enabling rapid and accurate detection of formaldehyde concentrations. Furthermore, the nanoscale size and tunable structure of these materials provide ample room for further optimizing sensor performance, including improving selectivity and reducing interference.
Zen et al. developed an innovative electrochemical sensor for formaldehyde gas
[56][92], featuring a platinum-coated screen-printed ultramicroelectrode wrapped in Nafion as the electrolyte. This sensor’s novelty stems from its detection mechanism, which uses high oxidation potential to transform formaldehyde into formic acid, thereby activating Pt catalyst sites. Employing square wave voltammetry, the sensor distinctly separates responses from platinum oxide reduction and formic acid oxidation, achieving a broad linear detection range with an impressive sensitivity down to 80 ppb.
Pd-based electrocatalysts
Palladium (Pd) nanomaterials exhibit high catalytic activity in the electro-oxidation of organic small molecules. Furthermore, compared to Pt, Pd-containing materials are particularly attractive due to their relatively lower cost and high tolerance to CO formation as a byproduct during formaldehyde oxidation. The use of nanoparticles with high electroactive surface areas can enhance the sensitivity of these materials to formaldehyde oxidation. Yi et al. employed a hydrothermal method, using PdCl
2, EDTA (ethylenediaminetetraacetic acid), and formaldehyde as precursors, to prepare unique three-dimensional porous Pd nanoparticles on a titanium (Ti) substrate
[57][96]. These nanostructured Pd electrodes exhibited outstanding electrocatalytic performance for formaldehyde oxidation in alkaline solutions. They demonstrated a low onset potential for formaldehyde electro-oxidation on the nanoPd electrode, approximately −0.85 V vs. the saturated calomel electrode (SCE), along with a significantly large anodic current density of 66.96 mA·cm
2.
Ag-based electrocatalysts
Nanoporous silver, owing to its high electrical conductivity, large surface area, high porosity, and cost-effectiveness, finds extensive applications in catalyzing the oxidation of small organic molecules. The catalytic activity of nanoporous silver catalysts is closely linked to their size, morphology, structure, and the physical environment in which they operate. Jeon et al. reported the synthesis of silver nanoparticles (Ag NPs) on graphene oxide functionalized with p-phenylenediamine (Px), resulting in the formation of GOPx-Ag nanocomposites
[58][106]. These nanocomposites were employed as catalysts for the direct electro-oxidation of formaldehyde, with an onset oxidation potential of −0.783 V. Cyclic voltammetry tests revealed oxidation–reduction processes (Ag↔Ag
2O) occurring on the electrode surface of Ag. In comparison to electrodes lacking Px, GOPx-Ag-modified electrodes exhibited a 57.3% increase in the oxidation response current for formaldehyde and a 159 mV negative shift in oxidation potential. This indicates that Px enhances the dispersion of Ag NPs, leading to higher electrocatalytic activity. The authors also explained that Px-functionalized graphene oxide provides N lone pair electrons. This functionalized GO is a dense two-dimensional (2D) material with unparalleled electrical conductivity.
3.2.2. Bimetallic-Based Formaldehyde Sensors
Bimetallic nanocatalysts have demonstrated significant advantages in catalyzing the oxidation of formaldehyde due to their unique properties. These catalysts typically consist of two different metals, leading to synergistic effects that enhance catalytic efficiency, selectivity, and stability. They often exhibit superior activity compared to single-metal catalysts, possibly due to improved electron transfer, increased active sites, and altered reaction pathways. Recent research has focused on optimizing the composition, structure, and size of bimetallic nanoparticles to further enhance their performance in formaldehyde oxidation. These advancements include the development of novel synthesis methods, understanding the nanoscale interaction between the two metals, and exploring various combinations of metals. Currently reported bimetallic nanocatalysts for electrochemical detection of formaldehyde include Pt-Ru
[59][109], Pd–Pt
[60][61][62][110,115,156], Pd-Au
[63][111], Ag-Pd
[64][112], Sn-Pt
[65][113], Cu-Pd
[66][67][114,116], Ni-Pd
[68][117], Pt–Ag
[69][118], and Cr-Pd
[70][119]. Due to Pd’s high catalytic activity and low susceptibility to poisoning during anodic oxidation, it is commonly used in alloy materials. Bimetallic catalysts comprising Pd and other noble metals have shown excellent catalytic performance in formaldehyde electro-oxidation.
Mardared et al. investigated the electrocatalytic performance of various compositions of copper–palladium (CuPd) thin film combinatorial libraries using cyclic voltammetry throughout the entire composition range of CuPd films
[66][114]. They found that Cu-7.5 at.% Pd exhibited the highest electrocatalytic activity for formaldehyde oxidation with a starting potential of −0.35 V and a current density of 1.81 mA∙cm
−2. The observed electrocatalytic enhancement at the optimal composition was attributed to the synergistic effects involving Pd concentration, surface properties, and electron density. Azizi developed a highly sensitive electrochemical sensor for formaldehyde detection using novel bimetallic nanoporous Pd-Cu-SBA-16/CPE (carbon paste electrode)
[67][116]. Bimetallic nanoparticles composed of palladium (Pd) and copper (Cu) were incorporated into SBA-16 using an electrochemical replacement reaction. This approach reduced the use of precious metal (Pd) while improving electrocatalytic performance. The electrochemical properties of the Pd-Cu-SBA-16/CPE formaldehyde oxidation sensor were thoroughly investigated using cyclic voltammetry, current analysis, and chronoamperometry. The sensor exhibited excellent electrocatalytic activity with high current density and low formaldehyde oxidation overpotential. Wang and Li et al. developed an electrochemical method for on-site detection of formaldehyde in food using Pt-Ag core–shell nanoparticles as the electrocatalyst
[69][118].
Metallenes, a cutting-edge topic in the field of materials science, have emerged as a novel class of two-dimensional materials composed of single layers of metal atoms, exhibiting unique physical and chemical properties
[71][72][73][157,158,159]. In the realm of electrocatalysis, metallenes hold tremendous potential owing to their high surface area, excellent electrical conductivity, and distinctive electronic structure
[74][75][76][160,161,162]. These attributes render them promising candidates for various applications, including catalyst supports
[77][78][163,164], energy storage
[79][80][165,166], and sensors
[81][82][167,168].
3.2.3. Transition Metals and Their Oxide-Based Formaldehyde Sensors
Transition metal materials have shown significant potential in electrochemical catalysis for formaldehyde oxidation. These materials, including nickel, copper, and other metal oxides, are capable of undergoing various oxidation states, which facilitates effective electron mediation required for formaldehyde oxidation. These catalytic materials can be obtained through various preparation methods, such as electrochemical deposition, vapor-phase deposition, chemical corrosion, in situ transformation, and more. They exhibit high catalytic activity for the electrochemical oxidation of formaldehyde and hold great promise in the construction of formaldehyde sensors. The following discussion will categorize these materials and their applications.
Nickel-based electrocatalysts
Nickel-based transition metal nanomaterials are the most widely used electrocatalysts for constructing electrochemical formaldehyde sensors. They are typically prepared using methods such as electrochemical deposition and chemical corrosion on different substrates. The mechanism of formaldehyde oxidation catalysis by these catalysts is well understood and often involves the conversion of Ni(II) to Ni(III) (Equation (
913)). Subsequently, Ni
(III)OOH species oxidize the ionized form of formaldehyde, CH
2(OH)O
− (Equations (1
04) and (1
15)), and Ni returns to its initial divalent state as Ni(OH)
2.
Ying et al. have introduced a sensitive and cost-effective flow injection analysis (FIA) method for detecting formaldehyde using an activated barrel-plated nickel electrode (Ni-BPE)
[83][120]. The mechanism of formaldehyde electrocatalytic oxidation on the Ni-BPE electrode in an alkaline medium at ambient temperature is discussed, involving the oxidation of formaldehyde by Ni
IIIO(OH) species. This method exhibits good linearity within the concentration range of 0.037 to 10 μg∙mL of formaldehyde, with a low LOD of 0.23 μg∙L. The method demonstrates excellent reproducibility and has been successfully applied to the determination of formaldehyde in commercial nail polish samples and drinking water.
Copper-based electrocatalysts
Copper exhibits various oxidation states, including Cu, Cu(I), Cu(II), and Cu(III), during the electrochemical process, which can act as effective electron-conducting intermediates for the oxidation of formaldehyde. Consequently, copper is widely employed in the construction of formaldehyde electrochemical sensors. Abnosi et al. investigated the electrocatalytic oxidation behavior of formaldehyde on a copper electrode in alkaline solutions
[84][135]. Their cyclic voltammetry studies revealed that the presence of formaldehyde resulted in an increase in peak current associated with Cu(III) oxidation, followed by a corresponding decrease in cathodic current.
Other studies have suggested that Cu(II) also plays a crucial role as a mediator in the oxidation of formaldehyde. Farhadi et al. prepared copper-porous silicon nanocomposite materials (Cu/PS) and explored their application in the detection of formaldehyde in electrochemical sensing
[85][139]. They first deposited copper nanoparticles onto etched porous silicon (PS) surfaces using an electrodeposition method, resulting in Cu/PS nanocomposite materials. Cu NPs/PS/SPCE exhibited significant electrocatalytic activity for the oxidation of formaldehyde at negative potentials. Compared to an unmodified electrode, the peak potential shifted approximately 0.7 V in the negative direction. Electrochemical tests suggested that the oxidation peak on Cu NPs/PS/SPCE was mainly associated with direct formaldehyde oxidation. Cu(II) ions generated during the forward scan on Cu NPs/PS/SPCE could effectively serve as intermediates for formaldehyde oxidation, resulting in a substantial increase in oxidation peak current. Tang et al. investigated the fabrication of a CuO/Cu/TiO
2 nanotube array (TNA)-modified electrode and its performance in formaldehyde detection
[86][141]. The CuO/Cu/TNA-modified electrode, crafted through an electrochemical approach, showed remarkable performance in catalyzing formaldehyde’s electrocatalytic oxidation.
Other transition metal oxides or sulfides, such as TiO
2/RuO
2 [87][144], MnO
2 [88][145], CeO
2 [89][149], Co(OH)
2 [90][147], MoO
x [91][146], ZnO
[92][93][150,151], and Ag
2S
[94][148], have also demonstrated the ability to catalyze the oxidation of formaldehyde. Bertazzoli et al. investigated the kinetics of formaldehyde oxidation in a flow electrochemical reactor with a TiO
2/RuO
2 anode
[87][144]. The reactor employed a titanium electrode coated with (TiO
2)
0.7(RuO
2)
0.3 and monitored the electrochemical degradation of formaldehyde solutions. The oxidation of formaldehyde, as well as the removal of total organic carbon (TOC) and chemical oxygen demand (COD), was primarily controlled by mass transfer. For a solution containing 0.4 g∙L
−1 of formaldehyde, the electrochemical degradation followed pseudo-first-order kinetics. The TiO
2/RuO
2 anode combination exhibited a higher rate of formaldehyde and formic acid oxidation compared to electrodes containing IrO
2. Nakayama et al. electrochemically deposited MnO
2 thin films onto glassy carbon electrodes using cathodic reduction from a KMnO
4 solution
[88][145]. The MnO
2 was of the hollandite-type structure and demonstrated catalytic oxidation of formaldehyde under mild pH conditions (pH 6.3 or 4.0).
3.2.4. Organic-Polymer-Electrocatalysts-Based Formaldehyde Sensors
Organic polymer electrocatalysts, typically formed by covalent bonds in organic structures, feature large conjugated systems, providing excellent electrical conductivity and stability
[95][96][97][98][176,177,178,179]. These cost-effective polymers are easily shaped and made through electrochemical polymerization or chemical redox methods. This process forms active sites on their surface or interior, enhancing their catalytic activity, particularly in oxidizing formaldehyde. Notable examples include polypyrrole
[99][152], polyaniline
[100][155], polydopamine
[101][153], and polyacrylonitrile
[102][154]. Advancements in synthesis and the exploration of new materials are expected to further improve these catalysts’ efficiency and broaden their applications.
3.3. Electrochemical Sensors Rely on Derivative Reagents
Formaldehyde, as an active organic molecule, exhibits specificity in its reactions with various organic functional groups, primarily due to its electrophilic nature and the presence of its carbonyl group. These specific reactions include the Lindlar reaction, which involves the formation of Schiff bases through the reaction of formaldehyde with amino compounds, the 2-aza-Cope reaction that leads to the rearrangement of imine structures, aldol reactions where formaldehyde adds to other carbonyl groups, and amine–formaldehyde reactions that result in the generation of compounds like urea. Leveraging these distinctive reactions, especially when combined with appropriate derivatization reagents, enables the development of highly selective formaldehyde electrochemical sensors.
Earlier research demonstrated that formaldehyde reacts with acetylacetone and ammonia to form a yellow substance, 3,5-diacetyl-1,4-dihydromethylpyridine (DDL), a reaction historically utilized for formaldehyde detection via spectrophotometry. In a novel approach, Silva found that DDL undergoes oxidation on unaltered glassy carbon electrodes, with an oxidation peak at 0.8 V, where formaldehyde is not electroactive
[45](Figure 14) [58]. This discovery suggests the potential for indirectly and selectively detecting formaldehyde electrochemically. The method showed a linear detection range from 0.4 to 40.0 mg∙L
−1 and a low detection limit of 0.13 mg∙L
−1, proving especially useful for quickly identifying formaldehyde in diverse samples.
4. Conclusions
(1) Diversity in Sensing Mechanisms: electrochemical sensors for formaldehyde detection are developed based on diverse principles like enzymatic reactions, usage of electrocatalysts, and specific chemical reactions, each offering unique advantages in terms of sensitivity, selectivity, and scope of application.
(2) Enzymatic Sensors: Primarily utilizing FDH, these sensors display high specificity and sensitivity towards formaldehyde. Advances in electrode material engineering and enzyme immobilization have notably enhanced their performance.
(3) Electrocatalyst-Based Sensors: Employing metals, metal oxides, and bimetallic nanocatalysts, these sensors are promising due to their high electrocatalytic activity and stability. Innovations in nanostructuring and surface modification have improved sensitivity and selectivity in formaldehyde detection.
(4) Chemical-Reaction-Based Sensors: Utilizing formaldehyde’s specific chemical reactivity, these sensors offer a highly selective detection method. Success depends on the precise selection of derivatization reagents and understanding underlying chemical interactions.