A colorimetric agarose gel for formaldehyde measurement based on nanotechnology involving Tollens reaction

Article abstract of DOI:10.1039/c4cc00914b

Gold nanoparticles (Au NPs) coupled with Tollens reagent were used for measuring formaldehyde. Au@Ag core-shell NPs were formed along with distinct color changes from pink to deep yellow. This colorimetric system was further immobilized into an agarose ge

Full text of DOI:10.1039/c4cc00914b

Showcasing research from Jing-bin Zeng’s Laboratory, State
Key Laboratory of Heavy Oil Processing and College of Science,
China University of Petroleum (East China)
As featured in:
A colorimetric agarose gel for formaldehyde measurement
based on nanotechnology involving Tollens reaction
Au NPs along with Tollens reagent were immobilized into agarose
gels for the colorimetric measurement of formaldehyde.
Au@Ag core/shell NPs were produced, and the thickness of
the silver shell was strongly dependent on the concentration of
HCHO, which further generated significant color changes for
visual readout.
See Jing-bin Zeng,
Zi-feng Yan, Xu-dong Wang et al.,
Chem. Commun., 2014, 50, 8121.
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A colorimetric agarose gel for formaldehyde
measurement based on nanotechnology involving
Tollens reaction†
Cite this: Chem. Commun., 2014,
50, 8121
Received 4th February 2014,
Accepted 6th May 2014
Jing-bin Zeng,*a Shi-guang Fan,a Cui-ying Zhao,a Qian-ru Wang,a Ting-yao Zhou,b
Xi Chen,b Zi-feng Yan,*a Yan-peng Li,a Wei Xinga and Xu-dong Wang*cd
DOI: 10.1039/c4cc00914b
www.rsc.org/chemcomm
Gold nanoparticles (Au NPs) coupled with Tollens reagent were delocalized conjugated functional groups (e.g., acetylacetone,
used for measuring formaldehyde. Au@Ag core–shell NPs were pararosaniline, chromotropic acid) are generally required as
formed along with distinct color changes from pink to deep yellow. recognition units.4 Moreover, these approaches are not sensi-
This colorimetric system was further immobilized into an agarose tive enough, are susceptible to interference, require strong acid
gel, which was used for monitoring of gaseous formaldehyde.
or base, and/or are time-consuming.5 Recently, a colorimetric
method for measuring HCHO has been proposed by using a
Formaldehyde (HCHO) is widely used in industry as a bonding Nafion film saturated with [Ag(NH3)2]+ and ATP. However, the
agent, especially for building materials, such as wood furniture method required a long reaction time (1 h) and suffered from
and paint. However, the use of these materials has caused inadequate sensitivity.6 Suslick and co-workers fabricated a
the remaining HCHO concentration indoors to exceed certain sensitive HCHO colorimetric sensor array based on the reaction
threshold, which puts people’s health at potential risk. For between primary amine and HCHO. The sensor array was fully
example, HCHO has been found to be highly associated with reversible, but relied on a series of pH indicators and statistical
the sick building syndrome, such as eyes and nose irritation, data-processing.7 Thus, a reliable, easy-to-operate, sensitive
fatigue, headache and coughing.1 In addition, it has been and low-cost HCHO detection method is still highly desired
classified as one of the human carcinogens by the world health for daily use.
organization (WHO).2 The measurement of indoor HCHO is
It is well documented in textbooks that HCHO can react with
closely related to the safety of living, which emphasizes the Tollens reagent (the key component is [Ag(NH3)2]OH) to pro-
need for a simple, sensitive and reliable method for measuring duce a silver mirror. Similar to a previous report,8 we also have
HCHO. Several approaches including chromatography,3a,b revealed that silver nanoparticles (Ag NPs) were produced when
electrophoresis,3c fluorimetry,3d enzyme-based biosensors3e, f the concentration of HCHO was relatively low (Scheme 1, down
and gas sensors based on metal oxide3g–i have been developed. row and Fig. S1a–c in the ESI†). There was an indistinguishable
These approaches are sensitive and reliable, but rely on the use color change from colorless to slightly yellow along with the
of scientific instruments which are not easily accessible by the appearance of a surface plasmon resonance (SPR) band at 416 nm
public. At the same time, special techniques are required for
instrument operation and data assessment.
Colorimetric methods are alternative simple approaches for
measuring HCHO, but extraneous organic chromophores with
a State Key Laboratory of Heavy Oil Processing and College of Science,
China University of Petroleum (East China), Qingdao, 266555, China.
E-mail: xmuzjb@163.com, zfyancat@upc.edu.cn
b State Key Laboratory of Marine Environmental Science, Xiamen University,
Xiamen 361005, China
c Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg,
93040 Regensburg, Germany. E-mail: xmuwxd@gmail.com
d Karlsruhe Institute of Technology (KIT), Institute for Biological Interfaces (IBG-1),
Hermann-von-Helmholtz-Platz, 76344 Eggenstein-Leopoldshafen, Germany
† Electronic supplementary information (ESI) available: Experimental procedures
Scheme 1 Schematic diagrams of the Tollens reaction with (upper row)
and Fig. S1–S13. See DOI: 10.1039/c4cc00914b
and without (down row) Au NPs.
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(Fig. S1d, ESI†). As shown in the upper row of Scheme 1, when Au
NPs were introduced in this classic reaction, the generated silver
nanoshells were coated onto Au NPs, leading to the formation of
Au@Ag core–shell nanostructures. The shell thickness of the
produced NPs was strongly dependent on the concentration of
HCHO. The change of shell thickness was accomplished with a
distinct color change from pink to deep yellow, which can serve as
a colorimetric approach for measuring HCHO.
The Au NPs were synthesized by reducing the HAuCl4 with
trisodium citrate, and the as-synthesized Au NPs exhibited
spherical shapes and uniform sizes (13.0 Æ 1.5 nm diameter,
Fig. S2 in the ESI†). The sole addition of Tollens reagent or
HCHO into the suspension of Au NPs did not lead to obvious
spectral or color changes (Fig. S3 in ESI†). In contrast, the
addition of incremental amounts of HCHO into the mixture of
Au NPs and Tollens reagent led to apparent color changes from
pink, to orange, and finally to deep yellow (Fig. 1a). These distinct
color changes were obtained in several minutes (Fig. S4, ESI†) at
room temperature (25 1C), which provides a rapid and simple way
to measure HCHO without using instrumentation and chromo-
phores. The lowest eye-distinguishable concentration can be as
low as 100 nM (3 ng mLÀ1).
A UV-Vis spectrometer was used to monitor the spectra
during the addition of HCHO into the mixture of Au NPs and
Tollens reagent. Fig. 1b shows that the SPR band of Au NPs
peaked at 520 nm blue-shifted, along with the emergence of a
second SPR band at around 410 nm, which implied the for-
mation of core–shell NPs.9 The absorbance at 410 nm increased
with the incremental concentration of HCHO (Fig. 1b). There
was a good linear relationship between the A410/A520 ratio and
HCHO concentrations in the range of 0.1–40 mM (Fig. 1b, inset),
which also enables a quantitative readout using UV-Vis spectro-
scopy. The lowest detectable concentration was found to be
50 nM (1.5 ppb). To the best of our knowledge, this is the
smallest LOQ for HCHO based on colorimetric methods.1b,4h,5,6,10
Fig. 2 Representative (a) TEM, (b) HR-TEM and elemental maps for the
core (c) and the shell (d) of the obtained nanoparticles.
The relative standard deviation of the assay is less than 1.6%
(Fig. S5, ESI†).
The structure and chemical composition of the produced NPs
were characterized using transmission electron microscopy (TEM).
Fig. 2a shows that most of the NPs were spherical and exhibited
inhomogenous electronic density with a darker central part and a
lighter outer part. A high-resolution TEM (HR-TEM) image (Fig. 2b)
clearly displays a spherical NP with apparent different optical con-
trast in the core and in the shell, respectively. Energy-dispersive
X-ray elemental analysis was used to map the element distribution
in a typical nanoparticle. As depicted in Fig. 2c and d, the elemental
composition of the core was gold, and that of the shell which
surrounded the gold core was silver, verifying the formation of
Au@Ag core–shell structures. The possibility of forming alloys was
excluded by the fact that nitric acid dissolved the Ag shell alone but
barely affected the Au cores (Fig. S6 in ESI†).
To get more insight into the mechanism of the approach,
TEM was used to measure the dimension of Au@Ag core–shell
NPs produced by the reaction with different concentrations of
HCHO. Fig. 3 indicates that the dimensional size of the Au@Ag
Fig. 3 TEM images and size distribution histograms of the Au@Ag core–
shell NPs generated by the Tollens reaction in the presence of Au NPs with
HCHO concentration of (a, d) 5 mM; (b, e) 25 mM; and (c, f) 100 mM. More
than 130 NPs were measured for each histogram.
Fig. 1 (a) Photographs and (b) UV-vis spectra of the colorimetric assay for
measuring HCHO at various concentrations. Inset in figure (b) shows the
linear relationship between the A410/A520 ratio and HCHO concentration.
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Au NPs into the classic Tollens reaction, Au@Ag core–shell NPs
were produced when exposed to HCHO. The thickness of the
silver shell was strongly dependent on the concentration of
HCHO, which further generated significant color changes for
visual readout. The approach was further transformed into a
solid matrix by using agarose gels to immobilize the Au NPs
and Tollens reagent. The produced agarose gels can measure
HCHO quickly, which is beneficial for immediate assessment
of potential risk and danger.
This work was financially supported by the National Nature
Scientific Foundations of China (No. 21105123, 21106185) and
the Shandong Young Scientist Awards (BS2012CL037) which
are gratefully acknowledged.
Fig. 4 Photographs of the agarose gel test strips for the detection of
HCHO with various concentrations.
core–shell NPs increased as the concentration of HCHO
increased, which corresponded well with the energy dispersive
X-ray and X-ray diffraction analysis results (Fig. S7 and S8 in
ESI†). In addition, as presented in Fig. S9 (ESI†), the plasma
resonance of Au@Ag core–shell NPs was highly dependent on
the thickness ratio of the silver shell to the gold core, and its
slight difference can lead to an observable change in absorp-
tion spectra and apparent color. This leads to the development
of a highly sensitive approach for HCHO measurement.
The selectivity of this assay was studied against common
indoor or outdoor gases, especially those with chemical reduci-
bility, such as alcohol, ketone, aniline and phenol. The con-
centrations of interferences were set at least 100 times that of
HCHO. As shown in Fig. S10 (ESI†), there is no obvious inter-
ference observed. Acetaldehyde, benzaldehyde and glucose were
tested to further evaluate the selectivity of our method. Fig. S11
(ESI†) shows that these aldehydes only produced little inter-
ference. A similar approach has been developed for colorimetric
detection of glucose,11 but our results indicate that glucose
exhibited limited response at room temperature even after the
reaction for 40 min. Such a good selectivity is attributed to the
fact that HCHO is the simplest molecule with virtually two
aldehyde groups,12 and so it can react with the Tollens reagent
quickly and leads to deposition of a thicker silver shell onto the
gold core. It should be noted that if all the tested aldehydes are
collected from gas phase using a diffusion based collector, the
discrimination would be even greater.13
The Au NPs along with Tollens reagent can be further
immobilized into the solid matrix for practical use. An agarose
gel was employed to embed the Au NPs and Tollens reagent,
since it is transparent, porous, and contains a large amount
of water to allow the occurrence of the Tollens reaction. The
obtained agarose gels were used for colorimetric detection of
aqueous HCHO. As shown in Fig. 4, the color of the agarose gels
changed from pink to yellow as the HCHO concentration
increased. The results (Fig. S12 in ESI†) also showed that the
agarose gels were able to measure the concentration of gaseous
HCHO as low as 80 ppb, which is the safety limit regulated by
WHO.7 The measurement range can reach up to 20 ppm which is
regulated by the Occupational Safety and Health Administration
as an immediately dangerous limit to life or health. The time
required for the visual measurement of HCHO at 20 ppm was less
than 1 min (Fig. S13, ESI†), which is beneficial for immediate
assessment of potential risk and danger.
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This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 8121--8123 | 8123