Redox transformation reaction for hierarchical hollow Au–MnOOH flowers for high SERS activity

Abstract

Hierarchical metal–semiconductor hybrid (Au–MnOOH) nanocomposite flowers with a hollow spherical base and prickly tipped divergent petals have been obtained for the first time by facile one step redox reaction. The wet chemical method reports a direct growth of Au–MnOOH nanocomposites through a redox reaction between Mn(II)-acetate and HAuCl₄ in aqueous solution. A thermodynamically controlled synthetic strategy requires no foreign reducing agents, stabilizing agents or pre-treatment of the precursors, making the strategy a practical method for obtaining metal–semiconductor hybrid nanomaterials. Liberated HCl (related to HAuCl₄ concentration) from the underlying redox reaction selectively etches out MnOOH from the spherical composite flower base i.e., the nucleation zone. Thus the flower base becomes a hollow basket with the roughened Au nanoparticles (NPs) surface bearing “hot spots” within. Finally the hollow nanocomposite Au–MnOOH flowers with prickly tipped divergent petals increase SERS activity with thiophenol (TP) and 4-aminothiophenol (4-ATP) as probe molecules. We observe the effect of the metal–semiconductor hybrid nanomaterial for giant SERS signal enhancement in comparison to the individual components. The Au–MnOOH nanoflowers with 5 atomic% Au show the best SERS enhancement with the probe molecules down to the single molecular level (EF > 10¹⁵) due to charge transfer (CT) as well as electromagnetic (EM) effects. Hierarchical hollow Au–MnOOH nanoflowers become a stable deliverable for extremely sensitive and reproducible SERS studies.

---

Introduction

Surface enhanced Raman scattering (SERS) has played an important role to examine the molecules adsorbed onto metal surfaces since its discovery in 1974. The SERS signal enhancement is due to the surface plasmon–polariton near field effect generated by the interaction of a laser beam with a SERS-active substrate. The efficient coupling of such a plasmon-induced near field effect with the vibrational modes of the probe molecules adsorbed on the SERS-active substrate resulted in the enhancement of the Raman cross section of the adsorbates by several orders of magnitude. Thus SERS becomes an important analytical technique which can provide higher chemical sensitivity of detection with spectroscopic precision and shows enormous potential for trace chemical selectivity.^1–3 Recent study shows that single molecule detection is possible with SERS, which is of considerable interest to the researchers concerned with nanomaterials, analytical chemistry and biomedical applications.^4–6 The most widely used metals with the appropriate optical characteristics in the visible to near-IR are Ag and Au, although other metals^7,8 can also support SERS activity with different laser light. Under these conditions, enhancement of the Raman signal of 10⁶ can be obtained at ease. It is already established that huge SERS enhancement is the resultant effect of electromagnetic effect (EM) and chemical effect (CE). But chemical effect is generally thought to contribute a factor of ∼10², compared to ∼10⁷ for electromagnetic effect for the effective SERS enhancement.^9–12

Semiconducting oxide nanoparticles have also proved to be excellent in detecting low-concentrations of molecules through SERS effects.¹³ Then the enhancement of the Raman activities arises from the large increase in polarizability due to charge transfer from the molecule to the semiconducting nanoparticle. Little is known about how the oxide nanocomposites of variable shapes and size, solvent, or pH affect the observed Raman activity. The marked increase in the Raman activity of molecules adsorbed on Au–SiO₂,¹⁴ Au–TiO₂,¹⁵ Ag–CuO,¹⁶ is again due to the combination of electromagnetic effect (EM) and chemical effect (CE). Thus metal–metal oxide nanocomposite would be useful and efficient alternative substrates for SERS studies.

The dielectric-core/metallic-shell nanostructure can provide huge SERS signal enhancement, as it converges with the scattering electromagnetic field at the metal surface.¹⁷ However, most of the approaches used so far are associated with expensive and complex preparation steps. Other major drawbacks are with stability and reproducibility. For example, Si nanowires are usually grown through an expensive high-temperature vapour–liquid–solid route.¹⁸ Nanolithographic techniques can greatly increase reproducibility but it is also an expensive technique. As for conventional lithographic technique the resolution limit is ∼10 nm.¹⁹ So the decrease of Raman intensity is observed in this case due to the fact that the electromagnetic field is strongly enhanced when the gap between metallic nanoparticle junctions ranges from 1 to 2 nm. Moreover, considering the stability, Au is much more oxidation-resistant than Ag and thus Au occupies an excellent position for SERS application.²⁰ Though Au nanoparticles are stable against oxidation, but they often coated with surfactant or other capping agent out of morphology controlled synthesis. These capping agents pose barrier which inhibit the approach of probe molecules to the surfaces or introduce unwanted impurity signals in the Raman spectrum. For these reasons, in recent years, many research efforts have been directed to prepare stable, naked and uniform SERS substrates.

Herein, a unique approach to prepare a SERS substrate is proposed exploiting gold chloride and manganese acetate as raw materials. This synthetic strategy has a major advantage as it goes in one step without any foreign reducing agents, stabilizing agents, or pre-treatment of the precursors. The proposed synthetic method gives rise to a new redox mediated way to develop metal–nanoparticle-loaded semiconductor composite.²¹ Finally we examine the new composite nanomaterial, Au–MnOOH hollow nanoflower (with 5 atomic% Au) for the giant SERS enhancement with probe molecules down to the single molecular level (EF > 10¹⁵) where charge transfer as well as electromagnetic enhancement from sharp tips transpire huge enhancement of SERS signals. Hierarchical Au–MnOOH nanocomposite acts as an extremely stable SERS substrate for reproducible result and it becomes a deliverable. Literature study as well as our experimental observation confirms that composite metal–semiconducting nanometerial may be used as promising SERS substrate.

Experimental methods

Synthesis of Au–MnOOH nanoparticles

At first, aqueous solution of three different compositions of reaction mixtures were prepared separately in three different screw capped test tubes taking variable amounts (250, 50, 30 mg) of Mn(CH₃COO)₂·4H₂O in 5 ml water then 2 ml HAuCl₄ (0.02 M) was introduced drop by drop into each test tubes under magnetic stirring condition at room temperature. The reaction mixtures were subjected to heating under modified hydrothermolysis (MHT) condition. For all the above cases we obtained the composite nanoflowers with different morphologies. Initially colorless reaction mixture turned to bluish black and then to light brown which indicates the onset of the evolution of the Au–MnOOH nanoparticles. Now, the brown solutions were kept for 3 h under modified hydrothermolysis (MHT) condition for the evolution of uniform Au–MnOOH nanoflower. To carry out MHT reaction a simple homemade cubic wooden box (7 inch × 7 inch × 8 inch) was used. An electric bulb (100 W) provides the required heat. The hydrothermolysis was done in a screw cap 12 ml long (1.5 cm in diameter) test-tube which precipitated the nanocomposite as discussed elsewhere. After 3 h of heating under MHT condition we obtained morphologically pure nanomaterials. But taking 250 mg Mn(II) acetate and 1 ml HAuCl₄ (0.02 M) and keeping all other condition similar we always obtained wire like morphology. To remove the acetate completely, the as-obtained product was heated on a water bath for 2 h. During heating on the water bath we repeatedly washed the product (thrice) with distilled water at 30 min interval to obtain Au–MnOOH nanoflowers for SERS studies. The as-obtained Au–MnOOH composite material can be transformed into Au–MnO₂ by annealing at 350 °C.

Procedure for SERS measurement

Fresh stock solutions of thiophenol or aminothiophenol were prepared regularly in ethanol with variable concentrations (10⁻⁶ to 10⁻¹⁵ mol dm⁻³) by dilution of the stock solution. Solution volume for every set of incubated thiophenol or aminothiophenol (10⁻⁶ to 10⁻¹⁵ mol dm⁻³) was adjusted to 1 ml for the Au–MnOOH flower while employed as SERS substrate (100 mmol L⁻¹ in ethanol). But during the spectral measurement we have taken 30 μl dispersion of the incubated (4 h) solution on aluminium foil for collecting SERS as well as NRS signals. So, the probing volume remains same to make a comparative account for enhancement.

Results and discussion

We have chosen a facile redox reaction to obtain hollow hybrid nanocomposites (Fig. 1) as a deliverable for fruitful exploitation of SERS activity. The resultant material is stable, robust and aesthetically novel for attachment of certain probe molecules.

Fig. 1 FESEM images of the obtained Au–MnOOH composite nanomaterials with 5% Au at different time interval ((A and B) after 1.5 h, (C and D) after 3 h of reaction).

Characterization of the particles

XRD analysis. The phase and purity of the products were examined by X-ray diffraction (XRD) on PW3040/60 X-ray diffractometer with Cu Kα radiation (λ) 1.54178 Å, the operation voltage and current maintained at 40 kV and 30 mA respectively. The XRD patterns (before heating) of the composites with variable amount (8%, 5% and 1%) of gold are shown in Fig. 2. The XRD patterns of Au–MnOOH composites are indistinguishable even after the incorporation of different amount of gold. All the diffraction peaks of the product can be indexed to be Au doped MnOOH. The XRD peaks can be well assigned to the planes 210, 020, 012, 202, 220, 222, 420, 412, 032 and 602 for MnOOH (JCPDS no. 74-1632) and 111, 200, 220, 311, and 222 for Au, respectively. The relatively broader diffraction peaks suggest the smaller crystallite size of the Au-doped MnOOH nano-architecture. After calcinations at 350 °C the XRD patterns of the synthesized Au–MnO₂ nanocomposites are illustrated in Fig. S1.†

Fig. 2 XRD patterns of the synthesized Au–MnOOH composite nanomaterials with different amounts of Au.

XPS analysis. XPS is a surface sensitive measurement for the determination of chemical state of material. The metal–metal oxide interaction in the Au–MnOOH nanocomposite was studied using XPS. Fig. S2† shows the core level XPS spectra in the Au 4f (Fig. S2A†) and Mn 2p (Fig. S2B†) regions for Au–MnOOH nanocomposites. The position of XPS Au 4f peaks depends strongly on the chemical/electronic environment surrounding it. The peaks located at around 83.5 and 87 eV were assigned to the spin–orbit splitted components of the Au 4f level in the pure Au metal.^22–24 Observed binding energy of the Au 4f_7/2 peak is about 84 eV in Au–MnOOH material, suggesting that gold exists in the form of metallic Au. Major peak 641.2 eV is due to Mn 2P_3/2 of Mn(III)²⁵ (Fig. S2b†).

Raman, TG and XRD analysis for thermal stability. All the Raman spectra are collected with a Renishaw Raman Microscope, equipped with a He–Ne laser excitation source emitting at a wavelength of 632.8 nm. A Peltier cooled (−70 °C) charge coupled device (CCD) camera and a Leica microscope with 50× objective lens is used.

To know the stability of the composite we have examined the laser power dependent studies involving Au–MnOOH nanocomposite with variable amount of Au. Normal Raman spectra have been collected taking each Au–MnOOH composite nanomaterial (having 1% and 5% Au) separately with increasing laser power keeping fixed acquisition time (Fig. 3). As for Au–MnOOH nanocomposite having lower percentage of Au (1% Au) small peaks at 367, 387, 531, 621 cm⁻¹ and a broad peak at 558 cm⁻¹ have been observed at low laser power (0.09 and 0.45 mW) in our spectral window. Raman peaks at 554 and 620 cm⁻¹ corresponding to the stretching modes of MnO₆ octahedra for pure MnOOH.²⁶ Again with increasing laser power (0.9 mW) spectral feature disappeared for MnOOH due to lower amount of Au (1%) in the composite. Whereas for Au–MnOOH nanocomposite having higher percentage of Au, (5% Au) small peaks at 367, 387, 531 cm⁻¹ and a broad peak at 558 cm⁻¹ have been observed even with the increased laser power (0.09, 0.45 mW and even at 0.9 mW). So, Au–MnOOH nanocomposite having 5% Au remains stable against higher (0.9 mW) laser power compared to the Au–MnOOH nanocomposite bearing1% Au. This speaks for the improvement of thermal stability of the composite having higher proportion of Au against laser heating.²⁷ The transformation of MnOOH to Mn₃O₄ takes place during the acquisition of Raman spectra but at higher laser power which caused the transformation presumably via MnO₂. With increasing laser power (4.5 mW for the composite having 1% Au and 9 mW for the composite having 5% Au) we observed completely different spectral feature. Where the bands at 305, 356 and 644 cm⁻¹ indicates the formation of Au–Mn₃O₄ during the acquisition of spectra because of local heating of the samples, which is consistent with reported Mn₃O₄ (ref. 28) sample. So, for the formation of Au–Mn₃O₄ from Au–MnOOH nanocomposite it requires higher laser power for the composite having higher proportion of Au (5%). Thus it can be concluded that the thermal stability of the Au–MnOOH composite gradually increases with the increasing amount of Au in the composite which is proved beyond doubt. This result justifies the suitability of the composite nanowire for SERS studies in terms of thermal stability against laser heating.

Fig. 3 Effect of laser heating during Raman analysis of Au–MnOOH composite with variable laser power (lp) speaks for enhanced thermal stability of the composites with increasing amount of Au.

Again, thermal analysis as well as temperature dependent XRD comply with the above observation. Comparative DTA curves of pure MnOOH²⁹ and Au–MnOOH composite material in the same window (Fig. S3†) indicate a gradual shift of manganese oxide transition temperature towards higher value. The thermogram indicates that the manganese oxide transition temperature for pure MnOOH is much less than that of the Au–MnOOH composite.

Temperature dependent XRD result supports the Raman and TG analysis. To get the exact information about the thermal stability of the synthesized composite material we have done the temperature dependent XRD analysis in the ex situ fashion. Each time fresh batch of as prepared pure MnOOH and Au–MnOOH sample with 5% Au was annealed successively at higher temperature and then XRD analysis was done. One representative XRD result is shown in the Fig. S4† which clearly shows that the pure MnOOH is converted to a mixture of Mn₂O₃ and Mn₃O₄ even at 820 °C. Moreover, for the Au–MnOOH composite with 5% Au remain stable as Au–Mn₂O₃ till 920 °C. So, the Au incorporation enhanced thermal stability of the composite is confirmed from the Raman, thermal as well as temperature dependent XRD analysis.

TEM, HRTEM, EDX analysis. The composition of the synthesized product was further confirmed from Energy Dispersive X-ray Spectroscopy (EDX) analysis. Elemental mapping obtained from EDX analyses of the Au–MnOOH composite nanoflowers at different time interval is shown in Fig. S5, S6† and 4. Line mapping from the EDX analyses (Fig. S7 and S8†) for each element on the petal of Au–MnOOH flowery morphology shows that petals are also composed of Au–MnOOH composite.

Fig. 4 Elemental mapping (B–D) for each element from the Au–MnOOH flower with hollow (A) sphere base.

The EDX analysis confirms that all the synthesized products composed of Mn, O, and Au. We can vary the amount of Au concentration in the composite nanomaterials just by varying the precursor salt concentration. The amount of Au in different nanopetals is very consistent for a particular product (Fig. S7 and S8†). We observed that for nanoflower with hollow sphere base the distribution of Au NPs becomes higher within the hollow sphere (looks like a basket). This is may be due to the in situ produced HCl during the redox reaction between HAuCl₄ and Mn-acetate. The flowers are 2 μm in diameter and the width of the petals are within 20–50 nm. We observed that with increasing the percentage of Au in the composite nanoflower the diameter of the nanopetals become thicker and converted to nanoflowers composed of nanorods (Fig. S9†).

The TEM image of wet chemically synthesized nanoflower is presented in Fig. 5. From the critical fringe spacing analysis we can identify the interplanar spacing of both the MnOOH as well as Au in the composite materials (Fig. 5C–E). Interplanar spacing 0.146 nm was attributed to Au. MnOOH also bear an interplanar spacing ∼0.15 nm for 031 crystallographic planes close to 0.146. But from XRD pattern of the composite (Fig. 2) we observed a highly intense 220 plane from Au in comparison to weakly intense peak from 031 crystallographic planes of MnOOH. Moreover, from the area mapping in EDX spectra analysis considering a single petal of the nanoflower we observed homogeneous distribution of Au and MnOOH. So 0.146 nm interplaner spacing is more likely emerges from Au. The interplanar distance of fringes is 0.339 nm, corresponding to the 210 planes of tetragonal γ-MnOOH in the composite materials. Fig. 5F represents the SAED pattern of the nanoflower.

Fig. 5 TEM (A and B) HRTEM (C–E) and SAED (F) pattern of the synthesized composite nanoflower having 5 atomic percent Au.

Growth mechanism. We report the synthesis of morphologically different Au–MnOOH composite nanostructures with variable amount of Au just by varying the precursor concentration [HAuCl₄ and Mn(CH₃COO)₂] ratio during the reaction under MHT condition at 160 °C. The growth of Au–MnOOH composite nanoflower occurs through a series of intermediates which are characterized by FESEM, EDX, TEM and HRTEM analysis. At a very low concentration of HAuCl₄ (1 atomic% Au) we observe wire like morphology of the composite material. But an increasing amount of HAuCl₄ (5% Au) always leads to flowery morphology with sharp petals for the composite material. Still higher concentration of HAuCl₄ (8% Au) helps to evolve divergent short petals which grew outwardly from a common spherical base of the Au–MnOOH composite.

When we introduce HAuCl₄ solution into the solution containing Mn(CH₃COO)₂, always a bluish brown coloration is observed at the start. This is due to Mn⁺⁺/Au³⁺ redox mediated gold seed formation. The Au seeds serve as nucleation site for nanoflowers, composed of nanorods resembling petals. Murphy et al. observed that higher seed concentration leads to decrease the aspect ratio of Au nanorods.³⁰ The wires are shortened and assembled together surrounding the nucleating Au centers as the Au seed concentration increases. From the FESEM images, the petals of Au–MnOOH nanoflower having higher percentage of Au are with flat structures and wider diameters lead to decreased aspect ratio compare to the petal having lower percentage of Au. The reasons why the morphology of the petal is formed with different diameter is not yet completely understood. Presumably higher amount of HAuCl₄ produce larger amount of gold seeds those lead to decrease the aspect ratio through faster growth steps as the spheroidal particles gradually grew into petals.

To understand the composite Au–MnOOH nanoflower (with 5 atomic % Au) formation we studied the time dependent growth process. The growth mechanism is investigated by FESEM analysis in an ex situ fashion and shown in Fig. 6. The redox reaction resulted in a brown solid, precipitate slowly which was collected at different time (2 min to 3 h) intervals from the aqueous reaction mixture.

Fig. 6 FESEM images of Au–MnOOH composite nanomaterials at different time interval (A–F).

Generally controlled growth of seed particles is altogether an art of manipulating spheres into rods or faceting wires out of metallic NPs. We presume the same strategy for composite nanoflower formation. At the initial stage of the reaction, after 2 min we observe composite nanoflower resembling marigold (Fig. 6A and S10†) morphology. The flowery leafy petals are sheet like and after 20 min of reaction, only spheres and wires (Fig. 6B) in the composite material are distinctly observed. Initially MnOOH happens to remain as amorphous material as we observed only the strong peaks for Au(0) in the XRD pattern of the composite (Fig. S11†). Amorphous MnOOH containing marigold flower like morphology of Au–MnOOH is composed of sheets which is converted to the mixture of rods and particles of Au and MnOOH after 20 min (Fig. 7) and finally evolve rods after prolong heating under MHT presumably through oriented attachment mechanism.^31–33 After 20 min of heating, MnOOH slowly acquires crystallinity and we observe the attachment of Au and MnOOH particle from HRTEM analysis. The interplanar distance of fringes is 0.337 nm, corresponding to the 210 planes of γ-MnOOH in the composite materials. Whereas fringes at 0.235 nm, corresponding to the 111 planes of Au(0) (Fig. 7). Time dependent XRD analysis (Fig. S11†) also supports the observation and finally after 1.5 h we obtain dandelion flower like morphology with sharp tips (Fig. 6E). Here we observe highly crystalline phase of MnOOH (Fig. S11†).

Fig. 7 FESEM (A and B) and HRTEM (C–F) images of Au–MnOOH composite nanomaterials obtained at different time interval. Figure (A) shows the amorphous MnOOH containing marigold flower like morphology of Au–MnOOH is composed of sheets. Figure (B–F) indicates the attachments of rods and particles of Au and MnOOH.

Interestingly enough the spherical bases of the nanoflowers become hollow spheres just like a basket with the elapse of time where higher proportions of AuNPs with roughened surface have been found inside the spherical base (Fig. 4). The growth demonstrates the formation of Au–MnOOH composite nanowires as well as nanoflowers. But the amount of Au in different nanopetals remains consistently similar (Fig. S7†). The EDX analysis indicates the existence and distribution of Mn, O and Au elements in the Au–MnOOH nanocomposites (shown in Fig. 4, S5, S6, S7 and S8†).

The time dependent growth kinetics study provides the exact growth processes for the formation of hollow morphology. After 1.5 h of redox transformation reaction we observe the nanoflower with divergent sharp tips (Fig. 6E). But after 3 h we observed the hollow nature of the flower base due to etching by the in situ produced HCl (Fig. 4 and 6F). This etching of MnOOH from the composite flowers is slow and caused by the in situ produced HCl due to the reaction between HAuCl₄ and Mn(CH₃COO)₂. With the progress of the reaction HCl is produced as a byproduct which reductively etches out Mn(III) easily as Mn(II). Then water soluble Mn(II) passes into solution making the hollow morphology. The selective MnOOH etching presumably assists the formation of prickly tipped flowery petals. The spherical base is etched progressively to a great extent because of the trapped HCl leaving aside Au NPs “hot spots” with roughened surfaces. Thus the dandelion morphology (petals with higher aspect ratio) was obtained (Fig. S12†). The flowers are 2 μm in diameter and the width of the petals are within 20–50 nm. Higher HAuCl₄ (>5%) and longer MHT reaction time easily destroy the flowery association into wires and particles of the nanocomposite due to higher amount of HCl generation.

Furthermore we studied the reaction for the 1D growth of Au–MnOOH nanowire from FTIR studies. Quantitative desorption of CH₃COO⁻ takes place while marigold like morphology of Au–MnOOH composite nanoflower were obtained only after 10 min of MHT reaction. No peak due to acetate is observed even after 10 min of MHT reaction (Fig. S13†). So it is concluded that acetate does not influence the growth process.

SERS study. We chose to investigate the SERS activity of TP and 4-ATP as probe molecules onto Au–MnOOH substrate. The SERS spectra of thiophenol (TP) adsorbed at the tip of the Au–MnOOH composite nanoflower at various concentrations are shown in Fig. S14.† In order to understand the composite effect for the observed SERS signal enhancement, we have measured the DRS spectra of MnOOH and Au–MnOOH composite with different amount of Au and plotted the results in same window. An interesting feature of the Au–MnOOH nanoflowers is their broad surface plasmon resonance (SPR), as shown in Fig. 8. Generally sharp SPRs have been reported in SERS active materials,^34,35 in such situation careful tuning of laser wavelength becomes necessary in order to get the maximum SERS enhancement. Moreover, we have used 633 nm laser for SERS measurement which works well in the present case due the broad banded SPR. Again we observed that the absorbance of MnOOH gradually increases after 600 nm with increasing amount of Au into the MnOOH matrix. By comparing the SERS spectra (Fig. S14 & S15†) and the normal Raman spectrum of TP (Fig. S14†), we observed that there were obvious changes in both intensity and shift of several SERS bands of TP. The peak at 1092 cm⁻¹ due to C–C stretching mode moved to 1071 cm⁻¹ in the SERS spectra, and the intensity also increased. Similarly, another peak at 1583 cm⁻¹ contributed to C–C stretching moved to 1572 cm⁻¹. All of these changes can be interpreted that TP absorbs on Au–MnOOH nanoparticles through chemisorptions, resulting in changes in its structure.³⁶ We calculated the enhancement factor by comparing the intensity of all the high intense peaks in the SERS spectrum with those peaks obtained in the normal Raman spectrum by using a technique similar to that reported previously.³⁷

Fig. 8 DRS spectra of Au–MnOOH hollo flower (A), Au–MnOOH (B) wire and MnOOH only(C).

Assignment of vibrational Modes for thiophenol Raman peaks³⁸ has been shown in Table S1.† And the lowest detection concentration for TP was observed as low as 10⁻¹² M.

To have a precise idea regarding the orientation of the molecule, we estimate the apparent enhancement factors (AEFs) of some selected Raman bands using the relation we reported.³⁹

Accordingly,

AEF = σ_SERS [C_NRS]/σ_NRS [C_SERS] (1)

where C and σ represent the concentration and the peak area of the Raman bands measured from baseline.

The AEF values of the enhanced Raman bands at various concentrations of the adsorbate are shown in Tables 1 & S1.† A sample calculation for 1083 cm⁻¹ band has been added to the ESI Fig. S16.†

Table 1 Assignment of vibrational modes for 4-ATP Raman peaks and Apparent Enhancement Factors (AEF) of some selected Raman bands of the 4-ATP molecule adsorbed on Au–MnOOH substrate

NRS, 0.1M (cm^−1)	Symmetry	Assignment	SERS [10^−6 M] (cm^−1)	AEF × 10^6	SERS [10^−7 M] (cm^−1)	AEF × 10^7	SERS [10^−8 M] (cm^−1)	AEF × 10^8	SERS [10^−14 M] (cm^−1)	AEF × 10^14	SERS [10^−15 M] (cm^−1)	AEF × 10^15
466 (w)	a1	νCCC	—	—	—	—	—	—	—	—	—	—
635(ms)	a1	νCCC	—	—	—	—	—	—	—	—	—	—
1004	a1	νCC + νCCC	1007	1.15	1006	2.83	1007	2.14	1006	0.24	1004	0.6
	b2	δCH	—	—	1042	—	—	—	—	—	—	—
1083(vvw)	a1	νCS	1076	0.36	1074	1.11	1074	0.6	1077	0.1	1075	0.03
1178	a1	δCH	1180	0.1	1180	5.47	1178	3.30	1178	0.48	1178	0.26
	b2	δCH + νCC	1391	—	1390	—	—	—	—	—	—	—
1490	a1	νCC + δCH	—	—	—	—	—	—	—	—	—	—
1590(s)	a1	νCC	1578	0.51	1577	1.74	1572	1.1	1584	0.22	1573	0.11

---

We have evaluated the effect of composite material as well as closely spaced sharp tips for potential application as SERS substrates by using 4-ATP as a model Raman probe. Fig. 9 shows the Raman spectrum of 0.1 M 4-ATP and the SERS spectra of 4-ATP on the Au–MnOOH composite nanoflower on an aluminium foil. In one control experiment, we also collected the spectrum of the Au–MnOOH composite nanoflower in the absence of 4-ATP. The Raman signal of Au–MnOOH composite nanoflower does not show any band common to ATP probe. This helps immensely to examine the SERS of ATP molecule on the Au–MnOOH composite substrate. The normal Raman spectrum of 0.1 M 4-ATP is similar to that reported in the literature.⁴⁰ Compared to the spectrum obtained in the solid, the SERS spectrum obtained on the Au–MnOOH composite nanoflower shows distinct frequency shifts for some changes in band intensity. The νCS band shifts from 1092 cm⁻¹ to 1077 cm⁻¹ (Fig. S17†), and another frequency shift from 1598 to 1577 cm⁻¹ was also observed. Such observations clearly show that the –SH group of 4-ATP makes direct contact with the Au–MnOOH composite nanoflower surface by forming a strong Au–S bond.⁴¹ The Raman spectrum of 4-ATP on the Au–MnOOH composite nanoflower exhibited four b2 modes (CT effect) at 1577, 1435, 1389, and 1140 cm⁻¹ and one a1 mode at 1077 cm⁻¹, which is quite similar to those of 4-ATP absorbed on Au nanoparticles.⁴² In addition, there are significant differences in some bands compared to the others in Fig. S16† For example, relative intensity significantly increases for the band δ(CH) at 1140 cm⁻¹ in curve a, which belongs to a b2 mode.⁴³ It indicates that a greater extent of charge transfer phenomenon is involved when the flowerlike nanoparticle arrays are exploited with ATP molecule. Area mapping (Fig. S18†) on the synthesized composite nanoflower as a SERS substrate taking all the probe molecules show uniformity of SERS signal intensities. Au–MnOOH hybrid nanoflower shows huge SERS signal enhancement in comparison to the individual components, Au nanoparticle⁴⁴ and MnOOH nanowire²⁹ (Fig. 10).

Fig. 9 NRS spectrum of ATP from 0.1 M in aqueous solution and SERS spectra of 4-ATP adsorbed on Au–MnOOH composite nanoflower at different concentrations of the adsorbate for λ_exc = 632.8 nm.

Fig. 10 Comparative SERS spectra of TP (10⁻⁸ M) adsorbed on (A) MnOOH, (B) 55 nm Au nanoparticle and (C) Au–MnOOH composite nanoflower for λ_exc = 632.8 nm.

Conclusions

In conclusion, a simple redox transformation reaction has been judiciously adopted here to obtain Au–MnOOH composite nanoflower under modified hydrothermal (MHT) reaction condition at ∼160 °C. The progress of the wet chemical reaction between Mn(CH₃COOH)₂ and HAuCl₄ is driven by thermodynamics under surfactantless and templateless condition. In situ produced HCl as a reactant selectively etch out MnOOH reductively from the nucleation zone resulting in hollow flower base. The composite hollow flowers bear prickly tipped Au–MnOOH petals with roughened Au nanoparticle enriched spherical bases, a gift of the redox transformation reaction. The synthesis is scaled up and become an excellent deliverable SERS substrate for reproducible results. Again, the sensitivity of SERS signal down to single molecular level is accomplished for an enhancement factor (EF) of ∼10¹⁵ for the employed probe molecules.

Acknowledgements

BRNS Bombay, CSIR, DST New Delhi for financial assistance and Indian Institute of Technology, Kharagpur for research facilities.

References

1. D. Bhandari, S. M. Wells, S. T. Retterer and M. Sepaniak, J. Anal. Chem., 2009, 81, 8061.
2. M. G. Albrecht and J. A. Crieghton, J. Am. Chem. Soc., 1977, 99, 5215.
3. K. Kneipp, H. Kneipp, I. Itzkan, R. R. Dasari and M. S. Feld, Chem. Rev., 1999, 99, 2957.
4. S. Nie and S. R. Emory, Science, 1997, 275, 1102.
5. H. Xu, E. J. Bjerneld, M. Kall and L. Barjesson, Phys. ReV. Lett., 1999, 83, 4357.
6. M. D. Porter, R. J. Lipert, L. M. Siperko, G. Wang and R. Narayanan, Chem. Soc. Rev., 2008, 37, 1001.
7. H. Zhang, X. Xia, W. Li, J. Zeng, Y. Dai, D. Yang and Y. Xia, Angew. Chem., Int. Ed., 2010, 49, 5296.
8. M. E. Abdelsalam, S. Mahajan, P. N. Bartlett, J. J. Baumberg and A. E. Russell, J. Am. Chem. Soc., 2007, 129, 7399.
9. L. M. Liz-Marzan, M. Giersig and P. Mulvaney, Langmuir, 1996, 12, 4329.
10. B. Nikoobakht and M. A. El-Sayed, J. Phys. Chem. A, 2003, 107, 3372.
11. J. Zhang, X. Li, X. Sun and Y. Li, J. Phys. Chem. B, 2005, 109, 12544.
12. W. E. Doering and S. Nie, J. Phys. Chem. B, 2001, 106, 311.
13. A. Musumeci, D. Gosztola, T. Schiller, N. M. Dimitrijevic, V. Mujica, D. Martin and T. Rajh, J. Am. Chem. Soc., 2009, 131, 6040.
14. M. Roca and A. J. Haes, J. Am. Chem. Soc., 2008, 130, 14273.
15. X. Li, H. Hu, D. Li, Z. Shen, Q. Xiong, S. Li and H. J. Fan, ACS Appl. Mater. Interfaces, 2012, 4, 2180.
16. Y. Wang, W. Song, W. Ruan, J. Yang, B. Zhao and J. R. Lombardi, J. Phys. Chem. C, 2009, 113, 8065.
17. H. Qi, D. Alexson, O. Glembocki and S. M. Prokes, Nanotechnology, 2010, 21, 085705.
18. M. Becker, V. Sivakov, U. Gosele, T. Stelzner, A. Gudrun, H. J. Reich, S. Hoffmann, J. Michler and S. H. Christiansen, Small, 2008, 4, 398.
19. E. C. Le Ru and P. G. Etchegoin, J. Chem. Phys., 2009, 130, 181101.
20. T. Wang, X. Hu and S. Dong, Small, 2008, 4, 781.
21. Y. Wang, P. C. Sevinc, Y. He and H. P. Lu, J. Am. Chem. Soc., 2011, 133, 6989.
22. S. Carrettin, P. Concepción, A. Corma, J. M. López Nieto and V. F. Puntes, Angew. Chem., Int. Ed., 2004, 43, 2538.
23. S. Carrettin, A. Corma, M. Iglesias and F. Sanchez, Appl. Catal., A, 2005, 291, 247.
24. A. Abad, P. Concepción, A. Corma and H. García, Angew. Chem., Int. Ed., 2005, 44, 4066.
25. S. Ardizzone, C. L. Bianchi and D. Tirelli, Colloids Surf., A, 1998, 134, 305.
26. C. M. Julien, M. Massot and C. Poinsignon, Spectrochim. Acta, Part A, 2004, 60, 689.
27. M. Pradhan, A. K. Sinha and T. Pal, Chem.–Eur. J., 2014 DOI:10.1002/chem.201304518.
28. F. Buciuman, F. Patcas, R. Craciun and D. R. T. Zahn, Phys. Chem. Chem. Phys., 1999, 1, 185.
29. W. Zhang, Z. Yang, Y. Liu, S. Tang, X. Han and M. Chen, J. Cryst. Growth, 2004, 263, 394.
30. C. J. Murphy and N. R. Jana, Adv. Mater., 2002, 14, 80.
31. S. Sarkar, S. Acharya, A. Chakraborty and N. Pradhan, J. Phys. Chem. Lett., 2013, 4, 3292.
32. H. Song, K.-H. Lee, H. Jeong, S. H. Um, G.-S. Han, H. S. Jung and G. Y. Jung, Nanoscale, 2013, 5, 1188.
33. Y. Cao, P. Hu and D. Jia, Appl. Surf. Sci., 2013, 265, 771.
34. T. R. Jensen, M. L. Duval, K. L. Kelly, A. A. Lazarides, G. C. Schatz and R. P. Van Duyne, J. Phys. Chem. B, 1999, 103, 9846.
35. S. M. Prokes, O. J. Glembocki, R. W. Rendell and M. G. Ancona, Appl. Phys. Lett., 2007, 90, 093105.
36. S. W. Han, S. J. Lee and K. Kim, Langmuir, 2001, 17, 6981.
37. A. D. McFarland, M. A. Young, J. A. Dieringer and R. P. Van Duyne, J. Phys. Chem. B, 2005, 109, 11279.
38. S. Li, D. Wu, X. Xu and R. Gu, J. Raman Spectrosc., 2007, 38, 1436.
39. M. Pradhan, J. Chowdhury, S. Sarkar, A. K. Sinha and T. Pal, J. Phys. Chem. C, 2012, 116, 24301.
40. Y. Wang, H. Chen, S. Dong and E. Wang, J. Chem. Phys., 2006, 124, 074709.
41. G. Wei, L. Wang, Z. Liu, Y. Song, L. Sun, T. Yang and Z. Li, J. Phys. Chem. B, 2005, 109, 23941.
42. T. Wang, X. Hu and S. Dong, J. Phys. Chem. B, 2006, 110, 16930.
43. M. Osawa, N. Matsuda, K. Yoshll and I. Uchida, J. Phys. Chem., 1994, 98, 12702.
44. G. Frens, Nature, 1973, 241, 20.

---

Footnote

† Electronic supplementary information (ESI) available: XRD, FESEM, EDX, FTIR, SERS and DRS spectra. See DOI: 10.1039/c4ra03544e

---