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3D-Printed Electrochemical Sensor Based on Graphite-Alumina
Composites: A Sensitive and Reusable Platform for Self-Sampling
and Detection of 2,4,6-Trinitrotoluene Residues in Environmental and
Forensic Applications
Ra´
ıssa R.D. Brum , Lucas V. de Faria , Natalia M. Caldas ,
Robson P. Pereira , Diego A. Peixoto , Samuel C. Silva ,
Edson Nossol , Felipe S. Semaan , Wagner F. Pacheco ,
Diego P. Rocha , Rafael M. Dornellas
PII: S2666-8319(25)00043-8
DOI: https://doi.org/10.1016/j.talo.2025.100441
Reference: TALO 100441
To appear in: Talanta Open
Received date: 20 January 2025
Revised date: 19 March 2025
Accepted date: 22 March 2025
Please cite this article as: Ra´
ıssa R.D. Brum , Lucas V. de Faria , Natalia M. Caldas ,
Robson P. Pereira , Diego A. Peixoto , Samuel C. Silva , Edson Nossol , Felipe S. Semaan ,
Wagner F. Pacheco , Diego P. Rocha , Rafael M. Dornellas , 3D-Printed Electrochemical Sensor
Based on Graphite-Alumina Composites: A Sensitive and Reusable Platform for Self-Sampling and
Detection of 2,4,6-Trinitrotoluene Residues in Environmental and Forensic Applications, Talanta Open
(2025), doi: https://doi.org/10.1016/j.talo.2025.100441
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©2025 Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/)
Highlights
• Laboratory-designed composite material for 3D-printed electrochemical sensors.
• On-site electrochemical detection of TNT in environmental and forensic samples.
• The adsorptive properties of Al2O3 greatly enhance the electrochemical response of
TNT.
• The proposed device serves as a sensor and sampler for TNT residues.
• The sensor is reliable for detecting nanogram levels of TNT in forensic scenarios.
3D-Printed Electrochemical Sensor Based on Graphite-Alumina Composites: A
Sensitive and Reusable Platform for Self-Sampling and Detection of 2,4,6-
Trinitrotoluene Residues in Environmental and Forensic Applications
Raíssa R. D. Bruma, Lucas V. de Fariaa,b*, Natalia M. Caldasa, Robson P. Pereirac, Diego A. Peixotod,
Samuel C. Silvad, Edson Nossold, Felipe S. Semaana, Wagner F. Pachecoa, Diego P. Rochae, Rafael M.
Dornellasa**
a Fluminense Federal University, 24020-141, Niterói-RJ, Brazil.
b Department of Analytical Chemistry, Institute of Chemistry, Federal University of Rio de Janeiro,
21941-909, RJ, Brazil.
c Military Engineering Institute, 22290-270, Urca-RJ, Brazil.
d Institute of Chemistry, Federal University of Uberlândia, 38408-100 Uberlândia, MG, Brazil.
e Federal Institute of Paraná (IFPR), Rua José de Alencar, 1080, 85200-000, Pitanga, PR, Brazil.
Corresponding authors: viniciuslucas82@yahoo.com.br* and rafaeldornellas@id.uff.br**
Abstract
The detection of explosives is of great importance in the forensic scenario. For this reason, we
proposed a lab-made graphite/alumina/polylactic acid (G/Al2O3/PLA)-based 3D-printed electrode
for 2,4,6-trinitrotoluene (TNT) electrochemical determination. The material was characterized by
infrared and Raman spectroscopy, scanning electron microscopy, and energy dispersion X-ray
spectra, indicating that G and Al2O3 were incorporated into the PLA matrix. The proposed electrode
combined with the square wave voltammetry (SWV) technique, demonstrated the ability to detect
TNT residues, where the first reduction process around -0.24 V was monitored for the analyses. The
developed electrochemical strategy supplied a linear range between 0.5 – 6.0 µmol L-1 and a
detection limit of 0.071 µmol L-1. The method's applicability was tested on real samples of tap,
lagoon, and seawater by direct analysis. Recovery values in the 100.9 to 105.8% range were
obtained, representing adequate accuracy. The lab-made electrode was also utilized as a sampler
to collect TNT residues on different surfaces, enabling the detection of TNT levels in the nanogram
range, and demonstrating the electrode's exceptional ability to detect trace amounts of the
compound. These results reinforce the device's potential as a viable alternative for fast, accurate,
and low-cost analysis in practical situations.
Keywords: 3D printing; conductive filament; fused filament fabrication; explosive; voltammetry;
forensic.
1. Introduction
The compound 2,4,6-trinitrotoluene (TNT) is an explosive widely employed in military
activities and industrial purposes including construction and mining [1,2]. Such substance and its
residues, if disposed of/handled inappropriately, can contaminate soils, water, and living beings due
to its relativity toxicity [3-6]. For that reason, the rapid qualitative and accurate quantitative analysis
of TNT is essential, not only for public health and safety but also due to its environmental impacts
[7,8]. Additionally, because the TNT is widely applied in terrorist practices, its analysis without
interference is of paramount interest for forensic purposes [9-11].
There is a vast literature reporting different techniques for determining TNT, such as mass
spectrometry [12,13], high-performance liquid chromatography (HPLC) [14,15], spectrophotometry
[16,17], Raman spectrometry [18,19], and chromatography [20,21]. Despite the aforesaid
techniques providing desired characteristics such as high sensitivity and accuracy, they present
drawbacks regarding portability and miniaturization (something increasingly required for analyses
outside the laboratory) [22,23]. Thus, electrochemistry stands out due to its rapid response, high
selectivity and sensitivity, low cost, and portability [24,25].
Associated with electrochemical techniques, 3D-printing technology has emerged as a
promising approach for the development of low-cost and disposable electrochemical sensors,
allowing the manufacture of customized sensors that can be designed for specific applications [26].
In this context, a widely explored strategy involves the production of bespoke conductive filaments
using accessible carbonaceous materials such as graphite (G) or carbon black (CB), castor oil as a
plasticizing agent, and recycled polylactic acid (PLA) (aligned with the principles of the circular
economy) as the thermoplastic component [27,28]. The development of polypropylene-based
materials has also emerged as a promising approach to broaden electroanalytical applications in
organic solvents [29]. Specifically for the sensing and self-sampling of TNT residues in a post-
explosion scenario, the Munoz’s research group proposed an alternative material based on G and
CB impregnated in a recycled PLA matrix [30].
Importantly, the strategy of producing lab-made filaments offers the analyst the flexibility to
develop materials with properties specifically tailored to the proposed application. In this regard, in
addition to carbonaceous materials, (nano)metallic materials can also be explored [10,31,32]. In
light of this, our research group has demonstrated that the incorporation of metallic materials
combined with G into thermoplastic matrices enhances their electrochemical performance [33-35].
Considering the self-sampling aspect of TNT residues, materials with adsorptive properties, such as
alumina (Al2O3), serve as excellent modifying agents. Additionally, this material offers chemical
stability due to its inert nature, helping to minimize degradation and/or passivation effects on the
electrode [34,36,37].
Therefore, considering the advantages of 3D printing and aiming to develop a cost-effective
analytical device with high electrochemical performance, this work focused on producing a 3D-
printed electrode using a lab-made (PLA)-based filament containing G and Al2O3 within the
polymeric matrix for the rapid, real-world, and point-of-need electrochemical determination of TNT.
2. Experimental
2.1 Chemicals and solutions
All chemicals employed in this study were of analytical grade and were used as received.
Standard and working solutions were prepared using deionized water, sourced from the Sartorius
Arium® Pro purification system (Göttingen, Germany), with a resistivity greater than 18.0 MΩ·cm.
Chloroform (99.8% v/v), acetone (99.5% v/v), ethanol (98% v/v), acetic acid (99.5% v/v), phosphoric
acid (85.0% v/v), and boric acid (98.0% w/w) were obtained from Synth (Diadema, Brazil). G and
Al2O3 were purchased from Sigma-Aldrich (St. Louis, USA). Potassium chloride (99.0% w/w) and
potassium ferricyanide (99.0% w/w) were acquired from Vetec (Rio de Janeiro, Brazil). Hydrochloric
acid (37.0% v/v) was sourced from J.T. Baker (Goiânia, Brazil), and perchloric acid (72.0% v/v) from
Hexis (São Paulo, Brazil). Caffeine (99% w/w), paracetamol (99% w/w) and 2-nitrophenol (98% w/w)
were obtained from Synth (Diadema, Brazil). Acetonitrile (99.8% v/v) was formulated by Vetec (Rio
de Janeiro, Brazil). Standard solutions (1000 mg L-1) of copper (II), and iron (III) were bought from
SpecSol (Brazil). Both PLA pellets and the acrylonitrile butadiene styrene (ABS) filament were
obtained from GTMax 3D (São Paulo, Brazil).
The background electrolyte for the electrochemical measurements initially consisted of 0.01
mmol L⁻¹ HCl. Subsequently, a 0.12 mol L⁻¹ Britton-Robinson (BR) buffer was employed with pH
values ranging from 2.0 to 10.0. The BR buffer was prepared using an equimolar mixture of acetic,
phosphoric, and boric acids, each at a concentration of 0.04 mol L⁻¹. For the pH adjustment, it was
employed a 1.0 mol L-1 sodium hydroxide (NaOH) solution. The stock solution of TNT (10.0 mmol
L⁻¹), sourced from the Military Engineering Institute (IME, Brazil), was prepared by dissolving the
solid in an appropriate volume of acetonitrile. For subsequent analysis, additional TNT solutions
were prepared by diluting the stock in the background electrolyte.
2.2 Preparation of G/Al2O3/PLA-based filament
The synthesis of the G/PLA/Al2O3 filament was based on the procedure described in a previous
work developed by our research group [34]. The control electrode was made in a percentage
proportion of 30:70 w/w (G/PLA) and the alumina-containing electrode was prepared in a
percentage proportion of 30:10:60 w/w (G/Al2O3/PLA), maintaining a ratio of 60% PLA to ensure the
material's printability. In brief, 18 g of PLA pellets were dissolved in 150 mL of chloroform/acetone
(1:3 v/v) at 70°C in a reflux system with constant stirring for 60 min. Next, both the conductive and
modifier materials, respectively G and Al2O3, were added. The system was maintained under the
same conditions for 2 hours. Afterward, the resultant material was solidified in ethanol, filtered,
and dried in an oven at 60°C. After 48 hours, the material was cut into 1 cm pieces, which were fed
into the Filmaq 3D extruder (Curitiba, Brazil) at 205°C, using maximum engine speed and a nozzle
diameter of 1.75 mm. The obtained filament was stored in plastic bags (Zip Lock) at room
temperature and protected from moisture (Figure 1).
Figure 1 - Preparation of G/Al2O3/PLA-based filament: (A) Addition of the reagents to the system;
(B) Solidification, filtration, and drying of the material produced; (C) Cutting of the material; (D)
Addition to the extruder and (E) Storage of the filament produced.
2.3 Manufacturing of customized 3D-printed electrodes
For the working electrode holder, cylinders measuring 40 mm in height and 8 mm in diameter
were additively manufactured using ABS filament on a GTMax3D FDM A2V2 printer (São Paulo,
Brazil) and designed with SolidWorks® 3D CAD software. A flexible copper wire was positioned
placed at the base of the holder to establish electrical contact with the potentiostat. The opposite
end was filled with the lab-made G/Al2O3/PLA-based conductive filament using a Sanmersen 3D pen
(Shenzhen, China). Excess material was removed by mechanical polishing with sandpaper of
different grits (600 and 1200), and the electrode surface was cleaned to eliminate any remaining
residue (Figure 2).
Figure 2 - Manufacturing of customized 3D printed electrodes: (A) Fabrication of the electrode base
on the 3D printer and addition of the copper wire; (B) Filling with the filament produced using a 3D
pen;(C) Removal of excess material by mechanical polishing; (D) Electrode ready for use. The inset
at the right shows a real image of the proposed electrodes.
2.4 Instrumentation and electrochemical setup
All electrochemical analysis (cyclic voltammetry (CV), electrochemical impedance spectroscopy
(EIS), differential-pulse voltammetry (DPV), and square-wave voltammetry (SWV)) were accessed
using a potentiostat/galvanostat Ivium CompactStat Technologies® (Eindhoven, The Netherlands),
model B09118, controlled by IviumSoft® software. A lab-made Ag|AgCl|KCl(sat.) [38] and a
commercial pencil graphite were used as reference and counter electrodes, respectively. The lab-
made G/Al2O3/PLA-based 3D-printed electrode was employed as a working electrode. The cell
vessel was prepared using a 5 mL glass beaker for the electrochemical studies.
Fourier-transform infrared (FTIR) spectra were obtained by PerkinElmer's Frontier MIR/FIR
spectrophotometer (Massachusetts, USA) in attenuated total reflectance (ATR) mode permitted by
the accessory purchased from Pike Technologies (Wisconsin, USA). Raman spectroscopy was
conducted with an Arion laser (wavelength of 532 nm under 2% incident power) using the LabRAM
HR Evolution microscope from HORIBA (Kyoto, Japan). Scanning electron microscopy (SEM) images
were acquired under 20 kV operation with the Vega 3 microscope from Tescan (Brno, Czech
Republic) using a secondary electron detector. Energy dispersion X-ray spectra (EDS) were obtained
from SEM images captured using the appropriate INCA X-Act detector from Oxford Instruments
(Abingdon, UK).
2.5 Analytical application in real water
To demonstrate the method’s applicability, different water samples, such as tap, lagoon, and
seawater, were analyzed through addition-recovery experiments. The tap water was collected in
our laboratory at the facilities of the Fluminense Federal University (Niterói, Brazil). The lagoon
water was collected in Araçatiba lagoon (Maricá, Brazil) and seawater was collected in Icaraí beach
(Niterói, Brazil). They were stored in plastic bottles in the fridge and used without any treatment.
2.6 Analysis of sampling TNT residue on different surfaces
For sampling analysis, 1.0 mg of TNT powder was distributed across a 25 cm2 area on
different surfaces, including granite, acrylic, wood, glass, and metal. For this, the powder was spread
until the material was not visible to the naked eye with gloved and naked hands to evaluate the
retention of the residue after handling. The proposed lab-made 3D-printed electrode (G/Al2O3/PLA)
was then used, passing it over the surfaces to collect the residue for analysis. Next, the electrode
was taken to the electrochemical cell which contained the appropriate background electrolyte for
the SWV analysis.
The banknote was hold with the naked hand for 5 seconds after spreading the TNT powder to
evaluate the contamination process through touching. For comparison purposes, blanks were made
following the procedure described, but before adding the TNT residue.
3. Results and discussion
3.1 Characterization of the composite material
The FTIR spectra of the two electrodes are shown in Figure 3A. The presence of PLA was
observed in both spectra through the vibrational modes of oxygenated groups, corresponding to
C=O stretching (1748 cm-1), C-O-H stretching (1353 cm-1), C-O-C stretching (1180 cm-1), and CH3/CO
rocking vibrations with overlapping C-O-C stretching (1127-1040 cm−1). An additional band is also
observed at 1452 cm-1, attributed to CH3 bending [39,40]. In a similar strategy employed by our
research group in a previous paper [33], Al2O3 microparticles presence was attested by the Al-O
stretching bands around 556-642 cm-1 [41] in the 30:10:60 wt% electrode. Due to the low dipole
momentum of the C=C bond that mostly composes the graphite structure, its vibrational modes are
weak in the FTIR-ATR spectrum. The direct interaction of the various oxygenated and non-
oxygenated functional groups of PLA promotes the overlapping, shifting, and broadening of the
vibrational bands of the carbonaceous material as well. Therefore, the interaction between both
materials made structural differentiation between the G/PLA support matrices difficult using the
FTIR technique, but clear modification with aluminum-based microparticles is demonstrated
through these results, due to the significant decrease in the PLA vibrational modes after the covering
with Al2O3 microparticles, that promotes an intense signal in the composite material.
Figure 3 - (A) FTIR-ATR e (B) Raman spectra of G/PLA (30/70 wt.%, black line) and G/Al2O3/PLA
(30/10/60 wt.%, cyan line) materials. The inset in B refers to the interval of 500 to 1100 cm-1.
The Raman spectra obtained for both samples are displayed in Figure 3B. In both the spectra,
the bands related to G present a stronger signal, due to the polarizability of the π electrons in the
C=C bond, causing high-intensity Raman scattering. The G band (1583 cm-1) is generated by
vibrations in the planar network of sp2 carbon atoms connected in a trigonal arrangement [42] while
the D band (1348 cm-1) is related to the breathing mode of the six-atom sp2 carbon rings [43] which
represents defects in the graphitic structure, arising due to the presence of heteroatoms and sp3
carbon atoms [44,45]. The inset showcases an increase in the signal of the lab-made G/Al2O3/PLA-
based electrode when compared to the G/PLA one, in the region related to the Al2O3 vibrational
modes (500 to 1100 cm-1) [46].
Using the ID/IG ratio, defects and structural changes in the composites were quantified. These
values were obtained through the deconvolution of the bands in the region in question (R2 > 0.99).
The ID/IG calculated for the G/PLA electrode was 0.65, while for the G/Al2O3/PLA was 0.30. The
predominant presence of PLA in the electrode (70-60% wt.) interferes with the degree of disorder
in the G structure, with the relatively aggressive chemical/mechanical processes involved in the
preparation of these electrodes disturbing the sp2 carbon network structural order. Interestingly,
the ratio reduced when the alumina was introduced to the system (0.30) if compared to the G/PLA
electrode (0.58), indicating the decoration of the G defects with Al2O3 microparticles and the
subsequential polarization due to small and highly positive ions of Al3+, which suggests that this
composition should be adopted when aiming for a less defective material.
The SEM images reveal that the non-Al2O3-based electrodes’ surface (Figure 4A) has a
compact and slightly wrinkled morphology, with well-integrated carbonaceous and polymeric
materials. The Al2O3-containing electrode exhibits a similar surface morphology, however,
showcases irregularly shaped Al2O3 particles onto the electrode surface (Figure 4B), with an average
size of 7.6 µm (Figure 4C). That way, the form and distribution of the Al2O3 particles incorporated
into the composite material, as well as the morphological characteristics of the well-dispersed
G/PLA matrix could be confirmed using SEM measurements.
Figure 4 - SEM images of the (A) G/PLA (30/70 wt.%) and (B) G/Al2O3/PLA (30/10/60 wt.%)
electrodes. (C) Relative frequency histograms of Al2O3 particle sizes.
The EDS spectra (Figure S1) depicted that carbon was the most abundant element, as the
electrodes were mainly composed of G and PLA. The oxygen peak observed in the spectra is
associated with the oxygenated functional groups found in PLA. The presence of alumina particles
in the electrode was confirmed by the aluminum peak detected around 1.5 keV in the EDS spectrum
(Figure S1B). The gold peak observed in the spectra is due to the sample metallization process,
performed to improve electrical conductivity during SEM measurements.
3.2 Electrochemical measurements
CV was used to evaluate the electrochemical performance of the proposed lab-made
G/Al2O3/PLA-based 3D-printed electrode toward TNT detection. For this, a potential window in the
(C)
range of +1.0 V to -0.9 V was applied, starting the scan at 0.0 V towards cathodic potentials (Figure
5 - cyan line). The voltammetric recording shows that, upon the proposed electrode, the TNT
presents a redox pair O1 and R4 between +0.2 V and +0.4 V, and three reduction peaks R1, R2 and
R3 at -0.24 V, -0.38 V and -0.59 V, respectively. In the absence of TNT, the electrode provides no
reduction processes apart from the reduction of oxygen at -0.6 V (Figure 5 - dash cyan line). The
non-Al2O3-based electrode provides the O1 and R4 redox pair in the same potential range, between
+0.2 V and +0.4 V, and the slight presence of the R1 peak also at a potential of -0.24 V (Figure 5 -
black line). In the absence of TNT, the electrode without Al2O3 shows no reduction processes (Figure
5 - dash black line).
Comparing the TNT’s electrochemical profiles upon both electrodes, it is possible to observe
a significant improvement in its electrochemical response for the Al2O3-based electrode, showing a
better-defined profile of the R1, R2, and R3 peaks, with a 20-fold increase compared to the R1 of
the free-alumina electrode. According to the literature, such aforesaid three processes are related
to the reduction of the nitro group (-NO2) to the amine group (-NH2). The mechanism proposed for
the reduction of TNT indicates that the process takes place in three consecutive steps, each involving
6 electrons and 6 protons in total (Figure 6) [47].
Figure 5 - CVs recorded in the presence of 0.1 mmol L-1 TNT in HCl 0.01 mol L-1 upon the G/PLA
(30:70, black solid line) and G/Al2O3/PLA (30:10:60, cyan solid line) electrode. The dashed lines
indicate the respective blanks. CV conditions: scan rate of 50 mv s-1 and step potential of -5 mV.
Figure 6 - (A) The mechanism of reduction of TNT and (B) Reduction of the nitro group to amine.
The origin of the process responsible for the presence of the O1 and R4 redox pair was
analyzed under the same conditions employed in the first analysis but with the scan in the direction
of the positive potential (Figure S2). It is possible to observe a dependence on the reduction
processes, considering that in the first cycle (black line) there is no presence of the redox pair peaks
(O1 e R4), which only appear in the second cycle (cyan line) after the reduction process. According
to the literature [7,48], the O1 peak corresponds to the oxidation of the hydroxylamine species, and
R4 corresponds to the reduction of the nitroso group, which occurs from portions formed in the
reduction of the nitro group of TNT (Figure 6B). Considering the acquired electrochemical profile of
TNT, the further studies were carried out by monitoring the R1 reduction peak.
The influence of pH on the TNT reduction process upon the proposed lab-made G/Al2O3/PLA-
based 3D-printed electrode was evaluated using BR buffer (0.12 mol L-1) as the background
electrolyte over a pH range of 2.0 to 10.0. The achieved voltammograms suggest that as the medium
becomes more alkaline, the definition of the finding peaks and the amount of reduction processes
is affected, becoming more difficult from an energy point of view, requiring more negative reduction
potentials (Figure S3A). It is also possible to note that at pH 2.0, a higher, well-defined, and sharp
R1 peak was achieved. Thus, the selected pH for the following analyses was 2.0.
In addition, a linear relationship was obtained between peak potentials and pH (Figure S3B),
which shows that the processes are pH-dependent. The R1 slope value of 44.2 mV pH-1 is close to
the theoretical value of 59.2 mV pH-1, indicating the same number of protons and electrons for this
process, which is in line with the mechanisms proposed in the literature (Figure 6) [47].
The background electrolyte was also evaluated. Solutions of 0.01 mol L-1 HCl, 0.01 mol L-1
HClO4, and 0.12 mol L-1 BR buffer, all at pH 2.0, were checked. The TNT reduction peaks were present
in all electrolytes, with no significant difference in definition and magnitude between them (Figure
S4). However, as the TNT reduction process depends on pH, to ensure that there is no variation in
the presence of other species, such as possible contaminants, it was decided to use BR buffer as the
background electrolyte in subsequent experiments.
Once the background electrolyte conditions had been optimized, the mass transport regime
of the TNT species (0.1 mmol L-1) was analyzed, varying the scan rate from 10 to 90 mV s-1 (Figure
S5A). Linear relationships were obtained between Ip vs. v (R=0.994, Figure S5B), suggesting
adsorption-controlled mass transport, and between Ip vs. v1/2 (R=0.996, Figure S5C), suggesting
diffusion-controlled mass transport. Then, for confirmation, the log Ip vs. log v graph was plotted
(R=0.997, Figure S5D), which showed a slope of 0.644 indicating a mixed process i.e., controlled by
diffusion and adsorption, since the slope of 0.5 indicates a pure diffusion process and 1.0 indicates
a pure adsorption [49].
3.3 Determination of TNT
To achieve more sensitive detection, a comparative study was carried out between the two
most widely studied pulse techniques for electroanalytical purposes, SWV, and DPV. Preliminary,
studies were carried out using 20 µmol L-1 of TNT in both techniques (Figure S6). SWV presented a
much better electrochemical response compared to DPV, with higher current intensity and better
peak definition being selected to develop the method.
Subsequently, aiming to obtain the best electrochemical behavior regarding peak definition and
analytical signal, the instrumental parameters of the SWV technique were judiciously optimized.
The modulation amplitude, frequency, and step potential were univariately optimized in the
presence of 20 µmol L-1 TNT with measurements in triplicate (n=3). First, the modulation amplitude
was evaluated in the range of 10 to 100 mV (Figure S7A). The peak currents increased as the
modulation amplitude increased up to 60 mV (Figure S7B). At higher values, there was not much
variation. Therefore, such value was chosen. Next, the frequency of pulse application was evaluated
between 10 and 100 Hz. It can be seen that, as the values increase, there is a considerable loss of
peak resolution and also a decrease in current (Figure S8A). The value chosen was 20 Hz because it
provided adequate peak definition and size, and a satisfactory current value with a smaller deviation
(less than 2%) (Figure S8B). Finally, the step potential was evaluated. For this purpose, it was varied
from -1 to -10 mV. It is possible to observe a better definition of the peaks and the growth of the
current as the values increase (Figure S9A). There is also a linear relationship between the values of
-2 and -10 mV, with a higher current value at -10 mV (Figure S9B). The value of -10 mV was therefore
chosen. The selected values are summarized in Table S1.
Under optimized conditions for using the proposed lab-made G/Al2O3/PLA-based electrode,
the detection of TNT was realized by SWV at different concentrations. Figure 7 shows that the
electrode can detect the three TNT reduction peaks even at low concentrations (0.5 µmol L-1 - red
line). It also possible to see, a linear behavior of the R1 reduction process with increasing
concentration (Figure 7B), in a first range from 0.5 to 6.0 µmol L-1 (R2 = 0.998) and in a second range
from 8.0 to 20.0 µmol L-1 (R2 = 0.996). The two linear ranges obtained can be refer to the two mass
transfer processes taking place in the system (Siqueira et al., 2023). The first range (0.5 - 6.0 µmol
L-1), related to the adsorption process, is favored by the presence of Al2O3 which acts as an
adsorption site on the electrode surface. At low concentrations, there is low competition for the
surface since the most accessible active surface area can be completely covered [50]. The second
range (8.0 - 20.0 µmol L-1), related to the diffusion process, is due to surface saturation at higher
concentrations. Knowing that carbon-based electrodes are rough materials, the analyte particles
adhere to the grooves, saturating the surface. This behavior has also been observed in other studies
using carbon-based sensors [50-52].
Based on the IUPAC recommendation, the limit of detection (LOD) was calculated [53,54]. The
LOD obtained for the R1 reduction process was 0.071 µmol L-1.
Figure 7 - (A) Baseline-subtracted SWV responses achieved for increasing concentration of TNT (0.5
to 20.0 μmol L-1) in 0.12 mol L-1 BR buffer (pH 2.0) as the background electrolyte. (B) Calibration
curves for the peak R1 upon the proposed lab-made G/Al2O3/PLA-based 3D-printed electrode. SWV
conditions: see Table S1.
A repeatability study was carried out to assess the precision of the electrode when applying
the method using a concentration of 3.0 µmol L-1 of TNT. For this, ten consecutive analyses were
performed employing the same electrode (Figure S10). For the R1 reduction process, a relative
standard deviation (RSD) of 0.80 % was obtained, demonstrating that the proposed electrode
provided proper precision. Additionally, no signs of typical problems such as fouling, making it an
efficient instrument for detecting TNT. A second study was conducted to investigate the effect of
temperature and humidity conditions on the sensor's performance. Ten analyses were also carried
out but on different days (Figure S11). An RSD of 1.13% was obtained for the R1 reduction process,
revealing that the electrode was not influenced by these conditions on the material's response,
since it was not stored under any special conditions, nor did it show any deterioration effect during
the analyses, so it could be reused. A reproducibility study was carried out to check the sensors’
manufacturing process. For this, three different lab-made G/Al2O3/PLA-based 3D-printed electrodes
were employed for the detection of TNT (3.0 µmol L-1) (Figure S12). For the R1 reduction process,
an RSD value of 0.54% was achieved, demonstrating that all the sensor production stages, from
material synthesis to 3D printing, were reproducible. Therefore, the sensor proved to be highly
efficient and suitable for analytical applications for determining TNT due to its consistent
reproducibility in all manufacturing stages. The analytical parameters achieved towards TNT
determination are summarized in Table 1.
Table 1 – Analytical performance achieved by SWV towards TNT determination (for R1 peak
analysis) upon the proposed lab-made G/Al2O3/PLA-based 3D-printed sensor.
Analycal parameters
Value
Linear range (µmol L-1)
0.5 – 6.0* / 8.0 – 20.0**
R2
0.998* / 0.996**
Intercept (µA)
-0.044* / -1.084**
Slope (µmol L-1 uA)
0.187* / 0.033**
LOD (µmol L-1)
0.071
% RSD (intra-electrode, n = 10)
0.80# / 1.13##
% RSD (inter-electrode, n = 3)
0.54
Values for the calibration curve in the *lowest and **highest concentration range; Values for RSD
carried out on the #same day and ##between 10 days.
3.4 Selectivity studies
To assess the selectivity of the proposed method using the lab-made G/Al2O3/PLA-based 3D-
printed electrode, the TNT determination analysis were carried out in the presence of possible
interfering agents. The nitroaromatic compound 2-nitrophenol, metals found in water (copper and
iron), and possible surface passivators (caffeine and paracetamol) were evaluated. All the analyses
were conducted in a 1:1 ratio between the interferents and the TNT, at a concentration of 3.0 µmol
L-1. Setting an interference limit of 10%, it can be seen that responses of between 96.6% and 104.6%
were obtained (Figure S13). It is considered that, under the conditions established as optimal, these
species do not interfere in the determination of TNT, being plausible to conclude that the developed
method is selective.
3.5 Application to real water samples
Three water samples from different sources were used for the real-world application:
laboratory tap water, lagoon water, and seawater. According to the analysis, under employed
experimental conditions, the samples showed no TNT signal or the concentration was below the
LOD of the method. For this reason, the samples were fortified at two concentration levels (1.0 and
2.0 µmol L-1) and the standard addition method was applied to minimize any matrix effect. There
was no prior preparation of the samples, a simple dilution in the background electrolyte was
required. Figures S14, S15, and S16 bring the findings for tap, lagoon, and seawater samples
respectively.
Table 2 provides the results achieved regarding real-world applicability. Recovery values in
the range of 100.9 to 105.8% were estimated, representing satisfactory accuracy, considering that
the samples may include other components.
Table 2 - Determination of TNT in real water samples using the proposed lab-made G/Al2O3/PLA-
based 3D-printed electrode considering the R1 process.
Sample
Added (µmol L-1)
Found (µmol L-1)
Recovery (%)
Tap water
1.00
1.06 ± 0.16
105.8 ± 7.9
2.00
2.03 ± 0.04
104.8 ± 3.8
Lagoon water
1.00
0.99 ± 0.03
102.6 ± 3.6
2.00
2.02 ± 0.14
100.9 ± 6.8
Seawater
1.00
1.04 ± 0.06
104.9 ± 5.4
2.00
2.10 ± 0.12
104.9 ± 6.2
3.6 TNT residue sampling
Screening techniques play an important role in field analysis, as they allow the quick and
efficient identification of the substance, without the need for complex materials. This type of
approach is useful where preliminary analysis is required, for example in the investigation of crimes
or accidents involving explosives.
Figure 8 shows the results for sampling on different surfaces. For each analysis, there was
the blank signal (black line) and the first measurement taken immediately after sampling (dashed
black line). In order to stabilize the signal, six measurements were taken, with the red line being the
last after stabilizing the signal. The next three measurements (green, blue and purple lines)
correspond to the addition of three aliquots of TNT standard solution aiming to confirm that the
analyte determined corresponds to the explosive.
Figure 8 - Baseline-subtracted SWV responses obtained for TNT residue detection after using as
sampler the proposed lab-made G/Al2O3/PLA-based 3D-printed electrode. First analysis (dash line),
after stabilization of the peak (red line) and standard additions of TNT (green, blue and purple) for
different surfaces: (A) Acrylic; (B) Granite; (C) Wood; (D) Glass; (E) Gloved hand; (F) Naked hand; (G)
Metal (H) Banknote. Experimental conditions: background electrolyte: 0.12 mol L-1 BR buffer (pH
2.0); SWV conditions: see Table S1.
It was possible to determine the mass of TNT sampled by the sensor using Faraday's Law of
electrolysis. The electrical charge (Q) of the R1 peak of the first measurement was applied to the
equation W = MM x Q / 96485,3329 x n, where W corresponds to mass, MM is the molecular mass
of TNT (227,13 g mol -1), n is the number of electrons involved in the process, and Q is the charge
[9,55]. According to the mechanism presented (Figure 6), a total of 6 electrons are involved in the
TNT reduction process, which is the value used to calculate the mass. Table S2 shows the calculated
mass values.
The values shown in Table S2 range from 0.75 to 2.95 ng. Among the surfaces used (acrylic,
granite, wood, glass, banknote, hand, and metal), granite had the lowest value collected due to its
higher retention compared to the other surfaces. The values also show that the sensor was able to
collect the material transferred to the banknote. Between the analyses of the glove and the naked
hand, the glove had the highest overall value, while the naked hand had the lowest value due to the
retention capacity of these surfaces. These values show that the sensor is capable of collecting
residues in the nanogram range and is very sensitive.
Considering real conditions where analysis may not be carried out at the same time as collection
due to the lack of instruments or the distance between the collection site and the analysis
laboratory, a storage study was carried out (Figure S17). Sampling was carried out as described
above on the granite surface, and the electrode was stored inside the electrochemical cell for 24
hours wrapped in transparent polyvinyl chloride (PVC) film without the supporting electrolyte to
evaluate the capacity of the electrode of G/Al2O3/PLA to retain the residues. After 24 hours, the
supporting electrolyte was added into the cell and the analysis was conducted. Following the initial
measurements, a TNT standard solution was added to compare the electrochemical reduction
profile.
Figure S17 shows that there was still an electrochemical response for the TNT reduction,
demonstrating that the electrode is capable of retaining the residues for at least 24 hours, indicating
that the manufactured electrode can be used as an analytical tool in forensic investigations.
Another repeatability study was carried out to analyze the precision of the electrode as a
sampler. For this, three analyses (n = 3) were performed for the same surfaces employing the same
electrode (Figure S18). A possible explanation for the variation in values is the porosity of the
material. Surfaces such as granite retain more waste due to their roughness, while acrylic, which
has a less porous surface, allows more waste to be captured. In addition, the direct contact of the
electrode with the surface can cause some morphological modifications, such as grooves, which can
lead to a change in the available sites and thus generate alterations in the analytical signal [9]. The
RSD values obtained for the R1 reduction process for each surface were summarized in Table S3.
Values below 5% were observed, indicating excellent precision during sampling and confirming that
the device maintained outstanding performance.
The analytical performance of the proposed lab-made G/Al2O3/PLA-based 3D-printed electrode
was compared with other electrochemical sensors reported in the literature for TNT determination
in terms of technique used, samples analyzed, and LOD (Table S4). Although some studies in the
literature show better results than the one presented in this work, it is important to note that many
of these methods involve more complex manufacturing steps, higher material costs, or more
rigorous experimental conditions, which may limit their practical applicability compared to the
proposed sensor.
In comparison to other 3D-printed carbon-based electrodes (G/PLA or G/CB/PLA) [30,52], it
is noteworthy that alumina has significantly enhanced the sensor's detectability, thereby improving
its performance for practical applications in diverse environments, such as the determination of TNT
in real-world aquatic matrices (e.g., tap water, pond water, and seawater). Furthermore, owing to
its versatility and robustness, it can also function as a sampler for collecting TNT residues at
explosion sites, as well as for mass detection in the nanogram range. It is also worth highlighting
that alumina is an inert material (it can be applied in different solvents (organic and inorganic) and
in a wide pH range), is cheap, and widely available in research laboratories, making it quite promising
in the manufacture of composite materials for sensors and electroanalysis [56,57].
4. Conclusion
The proposed lab-made G/Al₂O₃/PLA-based 3D-printed electrode, combined with the SWV
technique, demonstrated the ability to detect TNT, allowing the electrochemical profile of this
explosive to be characterized. Modifying the material with Al₂O₃ significantly improved the
sensitivity of the sensor, making it more efficient at identifying the analyte. In addition, the
reproducibility tests showed consistent results, confirming the reliability of the method developed.
The proposed sensor stands out for its portability, which facilitates analysis directly at the site of
interest, such as the detection of TNT residues in environmental water samples and on different
surfaces after explosion events. These results reinforce the device's potential as a viable alternative
for fast, accurate, and low-cost analysis in practical situations.
Acknowledgments
Rafael M. Dornellas and Lucas V. de Faria would like to thank the support from FAPERJ (E-
26/211.465/2019, E-33/201.429/2022, E-26/205.806/2022, 205.807/2022), and Rafael M. Dornellas
is thankful to CNPq (Universal 404044/2023-9). Edson Nossol is grateful to FAPEMIG (APQ-01207-
17) and the Brazilian Institute of Science and Technology (INCT) in Carbon Nanomaterials. Diego P.
Rocha is thankful to CNPq (Universal CNPq 401977/2023-4). The authors would like to acknowledge
the Multiuser Laboratory of Chemistry Institute at the University of Uberlândia for support involving
SEM experiments and the Multiuser Laboratory- LM-INFIS at the University of Uberlândia for the
acquisition of Raman spectra.
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Graphical abstract
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
