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Nanometer-thick Newton Black Film for Selective Formaldehyde Gas Detection Jingni Fu, and Luning Zhang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01254 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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Analytical Chemistry

Nanometer-thick Newton Black Film for Selective Formaldehyde Gas Detection Jingni Fu, Luning Zhang* School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China ABSTRACT: A fluorescence spectroscopic assay using Newton black film (NBF) for sensitive and selective detection of gaseous formaldehyde at room temperature is reported. The method relies on the Hantzsch reaction of formaldehyde with ammonium citrate and acetylacetone, plus a combination of the large surface area-to-volume ratio (5×108 m−1) and efficient uptake of gas by the nanometer-thick aqueous core of NBF. The assay has a limit of detection (LOD) of 4 ppb, a linear signal-to-concentration correlation up to 300 ppb of HCHO gas in the air, and a nonlinear monotonic increasing correlation in the range of 300 ppb−1.2 ppm. It is unaffected by relevant analytes such as acetaldehyde, benzaldehyde, acetone, and propionaldehyde. We also demonstrate the sensing of formaldehyde outgassing from a plywood sample using this method and the results agree with the factory specifications.

on a native bacterial NAD+- and glutathione-independent

Introduction Formaldehyde is widely used in the industry of

formaldehyde dehydrogenase for the direct detection of

automobile, textile and wood processing.

It is also an air

formaldehyde in air. Castro-Hurtado et al.24 demonstrated

pollutant,

blindness

and

the influence of the thickness of NiO sensing layer and the

carcinogenesis.4,5 The World Health Organization has

operating temperature for formaldehyde detection. Lin et al.4

1-3

which may cause allergy,

established limit of exposure to formaldehyde at a maximum

fabricated highly ordered vertically grown TiO2 nanotube

of 0.08 ppm averaged over 30 minutes, and the limit set by

arrays using the conventional electrochemical anodization

the Chinese Environmental Protection Agency is 0.06 ppm.6

process to detect formaldehyde gas. Hopkins et al.9

The Occupational Safety and Health Administration

developed an apparatus for sole formaldehyde detection

(OSHA) has also set a permissible formaldehyde exposure

through gas chromatography having an argon doped, pulsed

limit of 0.75 ppm for an 8-hour workday.7 The ability to

discharge

detect formaldehyde at very low concentration is crucial.

demonstrated

Many methods for detecting formaldehyde have been

functionalized polymer films doped with a pH indicator. The

developed8, including gas chromatography9-11, capillary

sensor arrays were imaged using an ordinary flatbed

helium a

ionization colorimetric

detector. method

Feng using

et

al.1

amine

liquid

scanner.1 Zhou et al.25 proposed a real-time formaldehyde

fluorometric

gas sensor based on the cataluminescence of formaldehyde

methods15,16, enzyme-based biosensors17-19, and metal oxides

and oxygen on the surface of V2Ti4O13, using a photon

12

electrophoresis , chromatography13,14,

high-performance colorimetric

and

. In Table S-1 we list some literature results to

counter for quantification. Motyka and Mikuška26 used a

compare the detection limit and concentration range realized

cylindrical wet effluent diffusion denuder for continuous

by different methods.

collection of formaldehyde from air into distilled-deionized

sensors

20-22

Methods in Table S-1 point to the focus on assays with enhanced sensitivity, simple operation, mild reaction conditions and high selectivity for formaldehyde gas

water. The concentration of the collected formaldehyde is determined fluorometrically by 2,4-pentandione method.26 Hantzsch reaction is an organic reaction between an

detection. For example, Knake et al. applied amperometric

aldehyde, two equivalents of a β-keto ester and a nitrogen

method to sense formaldehyde gas in an electrochemical cell

donor, and the reaction is quantitative for traces of

based on a gold coated Nafion membrane electrode.

formaldehyde under mild conditions.10 Thus, selective and

23

Achmann et al.17 developed an amperometric sensor based

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Page 2 of 8

sensitive fluorometric sensing methods for formaldehyde

spectral data for sensing standard samples (headspace of

based on Hantzsch reaction have been reported using

formaldehyde solution). In the end, we demonstrate sensing

β-diketones and ammonium acetate.

7,27,28,29,30

Bunkoed et al.

7

used the reaction of acetylacetone and ammonium acetate in sol-gels

for

the

spectrophotometric

detection

of

formaldehyde. Maruo et al.27 developed a sensor made of porous glass impregnated with both β-diketone and ammonium ions for formaldehyde sensing by UV-Vis absorption spectra. Li et al.28 demonstrated a novel regent, acetoacetanilide for the fluorometric determination of formaldehyde as low as 20 nM based on Hantzsch reaction. Current sensing methods based on the Hantzsch reaction relies on long reaction time for gas adsorption into liquid, thus for faster assay we need a system that can efficiently take up formaldehyde in the gas phase, which means high surface area to volume ratio.

of formaldehyde from a plywood sample outgassing. Experimental Section Chemicals. bromide

Triton

(CTAB),

X-100, sodium

cetyltrimethylammonium dodecylsulphate

(SDS),

ammonium citrate, acetone, formaldehyde solution (37%) and acetylacetone are from Aladdin Chemicals. Citric acid, sodium

hydroxide

and hydrochloric

acid

are

from

Sinopharm Chemical Reagent Co. Ltd. Acetaldehyde, propionaldehyde and benzaldehyde are purchased from Shanghai Titan Scientific Co. Ltd. All reagents are of the analytical grade. For Hantzsch reaction, a “stock solution” for gas sensing purpose is prepared. In 50 ml of water, we volumetrically dissolve 18.24 g of ammonium citrate, 170 µL of 0.45 mM citric acid solution, 20 µL acetylacetone and

Recently, Kanyanee et al.31 utilized a soap bubble to

300 µL Triton X-100, using small amount of hydrochloric

detect sub-ppm levels of SO2 by monitoring the bubble’s

acid to adjust its pH to 6.5. Thus the “stock solution” has

conductance after analyte uptake from the gas phase. They

0.01 M of Triton X-100 and 3.9×10−3 M of acetylacetone.

also used doped soap films for chiral pinene selective

Formaldehyde with specific concentrations in the headspace

transport.32 These methods suggest good potential for soap

of a cuvette is determined by Henry’s Law constants.36 A

film based assay, benefitting from the large surface area to

plywood sample of the Chinese E0 standard37 (≤ 0.5 mg/L,

volume ratio of the film. Inspired by these studies, we

GB/T 9846-2004) is stored for about 6 months out of the

recently developed assays using fluorescent pH probe in

factory before testing.

common black films (CBF) for the detection of ppm level ammonia and ppb level acetic acid.,33,34 In this paper, we report how Newton black films can be used to sense gaseous formaldehyde based on the Hantzsch reaction. The detection principle is shown in Figure 1. Our Hantzsch reaction is between formaldehyde and acetylacetone in the presence of ammonia citrate to form 3,5-diactyl-1,4-dihydrolutidine (DDL), a species with strong fluorescence35.

Figure 2. Diagram of cuvette with NBF and formaldehyde gas.

Instrument. A square stainless steel frame (10 mm × 10 mm) is used to draw a vertical film from the “stock Figure 1. Reaction equation of formaldehyde detection.

solution”, which eventually reaches an equilibrium thickness lacking colorful interference pattern.38,39 Depending on the

The unique features of our study are: first, a modified Hantzsch reaction relying on ammonium citrate (instead of ammonium acetate) to achieve faster reaction; second, nanometer-thick Newton black films (NBFs) for efficient formaldehyde gas uptake. This combination of NBFs with Hantzsch reaction gives us a simple, selective and sensitive fluorometric assay with a limit of detection of 4 ppb formaldehyde in the air. The linear response range is up to 300 ppb. After describing the experimental details, we present assay optimization, selectivity studies, and then

electrolyte concentration, a film can be either a CBF or a NBF (thickness 4–10 nm).40,41 In our case, a NBF is formed and its nanometer water-core is ready for dissolving formaldehyde gas, thanks to its higher solubility than most volatile organic compounds.42 After the film is drawn, it is put inside a quartz cuvette (4 cm × 4 cm × 4 cm) which contains gaseous formaldehyde as analyte. Inside the cuvette, NBFs exhibit good stability for hours. The diagram of the cuvette with NBF and gaseous formaldehyde


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Analytical Chemistry molecules is shown in Figure 2. A custom-built fluorescence

Finally, for measuring plywood sample outgassing, we

spectrometer with a 405 nm excitation laser is used. A 20 cm

compare a modified standard method (Test A) with our

focal length lens focuses the laser on the NBF at 45°

NBF-based assay (Test B). In Test A, we scale down the

incident angle. Fluorescence signal passes through a 450 nm

Desiccator method of the European Standard (ISO/CD

long-pass filter and is focused by a quartz lens onto the

12460−4:2016)46 to a smaller size so as to fit in our

entrance slit of a spectrograph (Andor Shamrock SR-303i-B)

instrument. In the standard Desiccator method, emission of

equipped with a cooled CCD detector at -60 °C. In assay

formaldehyde is determined by placing test pieces of known

optimization

concentration

surface area (1800 cm2) in a desiccator (volume: 11±2 L) at

formaldehyde solution into a bulk solution and then monitor

a controlled temperature and measuring the quantity of

the fluorescence change in a typical quartz cuvette (1.25 cm

emitted formaldehyde absorbed in a specific volume (300

× 1.25 cm × 4.5 cm).

mL) of water during 24 h.46 A glass crystallizing dish of

stage,

we

mix

different

Procedure. We first use FT-IR absorption (Nicolet iS10

inside diameter 115±1 mm is used as a container for water to

spectrometer) to characterize NBF’s thickness by measuring

absorb formaldehyde. In both tests, a plywood piece (3.1 cm

absorbance in the OH stretching region (3000−3600 cm ).

× 0.5 cm × 1.7 cm) which is manufactured to the Chinese E0

A cuvette with two CaF2 windows as the infrared beam path

standard37 (GB/T 9846-2004, ≤ 0.5 mg/L) is used. In an

is used. Conversion of absorbance at 3400 cm−1 into water

atmosphere with a mean relative humidity of 50±10 % and a

−1

core thickness is based on the work of Liao et al. Figure 3

temperature of 25±2 °C, the test piece reaches constant mass

shows the infrared spectrum of a NBF formed by the “stock

before testing. Then the plywood sample is partially sealed

solution” (without formaldehyde). The features within

using tape to expose a surface area of 10.1 cm2. Both Tests

2850−3000 cm−1 are from the C-H stretching vibration.44

A and B are carried out in a quartz cell (4 cm × 4 cm × 4

This NBF’s thickness is calculated to be 4.4±0.2 nm.

cm). In Test A, the formaldehyde outgassing from plywood

43

is absorbed into 1.684 ml of water (with 58 mm2 surface area) kept in the quartz cell for 24 h. In the end, the water containing absorbed formaldehyde is mixed with “stock solution” for fluorometric analysis. In Test B, the sample is kept at the bottom of the quartz cell. A fresh NBF prepared from the “stock solution” is put inside the quartz cell. Fluorescence signal is recorded continuously for 70 minutes. Data analysis. Fluorescent signal is quantified by the integrated peak area (Peak Area) of the peak centered at 515−520 nm. Without formaldehyde, the background fluorescence of the “stock solution” is defined as (Peak Area)blank. Upon reaction with the formaldehyde, an increase of the peak intensity occurs which is ∆(Peak Area). It is Figure 3. FT-IR absorption spectrum of the NBF formed by the micellar “stock solution”.

defined as: ∆(Peak Area) = Peak Area – (Peak Area)blank. Results and Discussion

For gas sensing, a freshly prepared NBF from the “stock

Assay optimization. We first verify the spectral features

solution” is put into a quartz cuvette (4 cm × 4 cm × 4 cm)

of DDL using bulk samples. In Figure S-1, we compare the

containing formaldehyde solution at the bottom of the

blank

cuvette. The assay relies on NBF’s gas uptake property.

formaldehyde with the solution that contains both reactants.

Analyte concentration in the cuvette headspace is controlled

Both the UV-vis absorption and the fluorescence excitation

by adjusting the bulk concentration defined by the Henry’s

and emission spectra indicate strong signal originated from

Law. A tight lid on the cuvette ensures gas phase

DDL molecules. We then explore the reaction parameters

equilibrium, which is also important for soap film stability.45

such as ammonium salt choice, pH of the “stock solution”,

Afterwards, fluorescence is continuously recorded for 40−60

surfactant ionic type, and reaction time as summarized in

minutes till signal stabilizes at the end of the Hantzsch

Figures S-2, S-3 and S-4. All tests are done in the bulk

reaction within the NBF.

solutions. In agreement with the study of Duan et al.47,

samples

that

contain

either

acetylacetone

or

Figure S-2 shows that the ammonium citrate system has



Page 4 of 8

shorter reaction time and larger signal compared with

Dong and Dasgupta49 as shown in equation (1), which gives

ammonium acetate. Therefore, the ammonium citrate system

the equilibrium concentration of gaseous formaldehyde at 25

is chosen for formaldehyde sensing. When solution pH is

°C:

5.5−6.5, the fluorescence intensity remains almost constant

HCHOaq. , M 7700HCHOgas , atm .

as shown in Figure S-3. Hence pH = 6.5 is used for all

For gas sensing, a series of formaldehyde solutions are

subsequent experiments. Three typical surfactants are

placed at the bottom of the quartz cuvette to maintain

chosen to compare their effects on the detection: cationic

specific concentrations of gaseous formaldehyde therein.

(CTAB), anionic (SDS) and nonionic (Triton X-100)

Then we rely on the intrinsic gas uptake property of NBFs

surfactants. The “stock solution” based on Triton X-100

for sensing. Figure 5a shows the fluorescence spectra as

exhibits the shortest reaction time (40 min) for all three

formaldehyde concentration increases from 0 to 1.1 ppm.

typical formaldehyde samples (1, 10 and 50 µM), as shown

The blank gives a weak fluorescence emission centering at

in Figure S-4. Thus Triton X-100 is used for preparing the

505 nm. Upon reaction with formaldehyde, a peak centered

“stock solution” and for the NBF assay.

at 520 nm starts to increase. The fluorescence signal linearly

Assay selectivity. For selectivity study, we mix relevant

(1)

increases with the concentration of formaldehyde in the

analytes benzaldehyde (a), acetone (b), propionaldehyde (c),

0−300 ppb range as shown in Figure 5b. In the inset of

and acetaldehyde (d) into the “stock solution” to reach 250

Figure 5b, we show the linear fitting result which has a

µM concentration. For parallel comparison, 25 µM of

formula of: y=1.0741x+6.5908 (R2=0.9980). To show the

formaldehyde is used. We measure the final fluorescence

assay’s

signal which is (Peak Area)S and compare it with (Peak

double-logarithm plot is used in the inset. According to

Area)blank of the blank, i.e., the “stock solution” without any

IUPAC recommendations, the limit of detection (LOD) is

analytes. All tests are done in the bulk. As shown in Figure

calculated using the formula15: LOD = 3δ / k, where δ

4, the ratio (Peak Area)S/(Peak Area)blank increases

represents the standard deviation of the Peak Area in blank

dramatically only for formaldehyde, even though its

experiments and k is the slope of the linear fitting curve in

concentration is only 10% of other analytes (25 µM vs. 250

Figure 5b. Based on the standard deviation of spectra, the

µM).

LOD for formaldehyde gas sensing is calculated to be 4 ppb.

Acetaldehyde

is

known

to

produce

response

at

low

analyte

concentration,

a

diacetyldihydrocollidin, however it lends no more than 1% interference to the final signal.48 Owing to the mild conditions and short reaction time of this assay towards formaldehyde, our results indicate that the system has high selectivity for formaldehyde.

Figure 4. Fluorescence signal change of the “stock solution” reacted with 250 µM of interfering analytes: benzaldehyde (a), acetone (b), propionaldehyde (c), acetaldehyde (d), and 25 µM of formaldehyde (e). Blank gives a value centered at 1.

Formaldehyde gas sensing. To prepare samples with known gas concentration, the vapor-liquid equilibrium data of formaldehyde49,50 are needed. We refer to the studies of

Figure 5. (a) Fluorescence spectra of the NBFs reacted with different concentration of formaldehyde gas, (b) ∆(Peak Area)


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Analytical Chemistry plotted against the concentration of formaldehyde in the gas

solution with specific vapor pressure, we shall see signal

phase. Inset: double-logarithm plot to show the 0−300 ppb

saturation towards the end of reaction. In Test B, the

range along with the linear fitting result (solid line).

fluorescence signal could increase linearly with time all the

In the 300 ppb−1.2 ppm range, a nonlinear monotonic increasing correlation is seen. Currently we have yet to

way to 24 h as in the initial 70 min if NBF continues to exist, since plywood gives off formaldehyde continuously.

develop a protocol to extend the linear response range

Using the first 70 min of data, we derive that the gas

beyond 300 ppb. In principle, the linear range of Figure 5b

phase concentration reaches 94±1 ppb (0.12 mg/m3) within

could be shifted to higher or lower concentration if we

the tested 55 min by comparing the results in Figure 6c and

optimize

concentration

Figure 5b. Assuming that ∆(Peak Area) in Figure 6c

combinations of reactants. Relatively long reaction time is

continues to increase linearly with time, as in the first 55

the other limit of our assay. We may decrease the reaction

min period, then formaldehyde gas concentration would

time using higher temperature or introducing turbulence in

eventually reach about 3.04 mg/m3 at 24 h. Multiply this

the fluid to facilitate species transport. However, such

number by the volume of the quartz cuvette, we get a total

approaches might render the NBF unstable.51

formaldehyde mass of 1.94×10−7 g during 24 h of

the

assay

using

different

outgassing. Suppose this amount of formaldehyde all

Plywood outgassing

dissolves in the 1.684 ml water in Test A, we will then have Two tests are performed for comparison. Test A (Figure 6a) is a miniature version of the Desiccator method (ISO/CD 12460−4:2016), and Test B (Figures 6b, c) is based on our formaldehyde gas assay described above. The red line in

a formaldehyde solution with a concentration of 0.115 mg/L, a value smaller by about 3% than 0.119 mg/L that is directly from Test A. Detailed calculations is summarized in the supporting information.

Figure 6a is the blank before sensing, and the black line represents the plywood sensing result after 24 h. In order to quantify the results in Test A, we need to know the exact concentration of formaldehyde in the bulk absorbing water. To do this, we perform a series of bulk reaction by mixing the “stock solution” with a set of formaldehyde bulk solutions, shown in Figure S-5. After comparing the ∆(Peak Area) value of the plywood with this working curve in Figure S-5b, we derive that the formaldehyde concentration in the aqueous solution in the absorption cell is about 3.97 µM , which is 0.119 mg/L.

Both measurements indicate that the plywood sample complies with the Chinese E0 standard37 (GB/T 9846-2004, ≤ 0.5 mg/L), which is consistent with the manufacturer specification. Chinese E0 standard37 is the same as Japanese

F*** standard (JIS A 1460, ≤ 0.5 mg/L)52. The formaldehyde limit for F*** rating of 0.5 mg/L was found to be equivalent to 0.06 mg/m3 in the European chamber test (method: EN 717-1) according to the research of Risholm-Sundman et al.53. The difference between Test A and B may originate from the different test conditions53 and the deviation of actual situation from the assumptions we

Figure 6b shows the fluorescence spectra of Test B during 70 min of reaction, after which the NBF breaks. As in Figure 6c, ∆(Peak Area) increases linearly with time during 15−70 min. The linear fitting function for this range is y=5609.1x−76215.4

(R2=0.9929).

Obviously,

NBF

supported by the frame does not last after 70 min in the presence of only plywood. It is likely due to the lack of water vapor to maintain film stability. Nevertheless, we may

made in Test B. We assume that the formaldehyde emitted from the plywood continues to increase at a steady rate during 24 h of testing, and the formaldehyde is completely dissolved in water. The actual sample may not follow exactly

these

assumptions.

Nevertheless,

a

positive

correlation is seen between our NBF assay and the modified Desiccator method of the European Standard (ISO/CD 12460−4)46.

assume that the reaction can continue up to 24 h as in Text A. This scenario is different from the gas sensing process shown in Figure S-6. If the sample is a formaldehyde



Figure 6. (a) Fluorescence spectra of the “stock solution” mixed with the water used for absorbing analyte in Test A and the blank, (b) the fluorescence spectra of the NBF with plywood sample in Test B over the period of 0-70 min, (c) the ∆(Peak Area) of the NBF in Test B plotted as a function of reaction time. The data between 15-70 min has been used for linear fitting (solid line). Figure S-5. (a) Fluorescence spectra of the “stock solution”

Conclusion We report a sensing system based on Newton black film

reacted with different concentration of formaldehyde in the

with a few nanometer thickness for the determination of

bulk. (b) ∆(Peak Area) plotted along with the concentration of

formaldehyde in the air based on Hantzsch reaction. The

formaldehyde solution. Inset shows the linear fitting function

assay is unaffected by other relevant analytes such as

below 75 µM.

benzaldehyde, acetone, propionaldehyde, and acetaldehyde.

Figure S-6. Change of ∆(Peak Area) along with the reaction

For formaldehyde in the gas phase, we obtain a detection

time in 0.10 ppm formaldehyde gas sensing.

range of 4−300 ppb with a LOD of 4 ppb. The formaldehyde

AUTHOR INFORMATION

gas detection range of this system covers what is important for occupational health. Furthermore, we apply the method

Corresponding Author

for plywood formaldehyde outgassing detection and our

*E-mail: [email protected]

results are consistent with factory specifications, showing the assay’s potential for real sample measurement.

ACKNOWLEDGMENT This work is funded in part by: Fundamental Research Funds

ASSOCIATED CONTENT

for the Central Universities (Tongji 1380219126), Shanghai

Supporting Information

Science and Technology Commission (14DZ2261100) through

The Supporting Information is available free of charge on the

Sustainability, and Ministry of Science and Technology (No.

ACS Publications website. Table S-1. Comparison of different methods for formaldehyde detection.

2012YQ22011303).

REFERENCES

Figure S-1. Absorption spectra (a) and fluorescence excitation and emission spectra (b) of Solutions 1, 2 and 3. The compositions of Solutions 0, 1, 2, and 3 are shown in the figure. All solutions have pH=6.5 adjusted with hydrochloric acid. Figure S-2. Fluorescence emission (Peak Area) of the ammonium citrate and ammonium acetate systems reacted with formaldehyde as a function of reaction time. Initial solution contains 10−5 M of formaldehyde, 0.01 M of Triton X-100, 3.9×10−3 M of acetylacetone, and 1.5 M of ammonium salt. Figure S-3. Fluorescence (Peak Area) of the “stock solution” reacted with different concentrations (5×10−5 M, 10−5 M and 10−6 M) of formaldehyde at different solution pH. Figure S-4. Fluorescence (Peak Area) of CTAB, Triton X-100 and SDS micellar “stock solution” reacted with (a) 10−6 M, (b) 10−5 M and (c) 5×10−5 M formaldehyde as a function of reaction time.

Shanghai Key Laboratory of Chemical Assessment and

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