The independence referendum should have tight spending limits to prevent it being "bought and sold for English gold", according to the First Minister.

Alex Salmond offered some "free advice" to the US on democracy and campaign spending limits during his visit to Chicago for the Ryder Cup.

Speaking to local radio station WBEZ, he said there is still "a persuasion job" to be done to convince people in Scotland to vote for independence.

He said "bigness" does not guarantee economic security, and predicted Scotland's fortunes as a small country could be boosted by an oil price surge as the world economy recovers.

Mr Salmond also revealed that he has donated a copy of a 15th century Act of the Scottish Parliament to a Chicago golf club, which restricted golf in favour of archery training to defend Scotland against the English.

WBEZ suggested that a deciding factor in the referendum may lie in "how snotty the British get about this".

Mr Salmond replied: "I love the word snotty. I've heard many descriptions about what is going to decide the referendum but snottiness is not one (of them).

"We're two years out from this referendum and I think there is a persuasion job to be done, but I think we're capable of having that persuasion campaign.

"And I think given the circumstances we'll face, and the opportunities that we have, I'm confident that we'll get a Yes vote for independence."

Mr Salmond was due to return to Scotland today to prepare for his forthcoming meeting with Prime Minister David Cameron to conclude their negotiations on the referendum process.

The wording and number of questions is expected to be finalised, alongside the possible inclusion of 16 and 17-year-olds in the vote and campaign spending limits.

In the Chicago radio interview, Mr Salmond borrowed a phrase from Robert Burns to emphasise the need to ensure either side doesn't "buy the ballot".

He said: "It's rather important to have limits on campaign spending and that applies to elections and referendums. Why? Because you don't want someone to buy the ballot.

"And certainly if you're coming to a vote on independence, you don't want it 'bought and sold for English gold'.

"We wouldn't want that to happen. A tight campaign limit, a fair campaign limit that is the same for both sides of course, is very important in a referendum.

"And if I could offer this free advice to the American people, I think given the democracy that is so evident everywhere in America and the wonderful free expression of opinions and ideas, if there was a way to do that without spending billions of dollars then I think a lot of people would like that as well."

The SNP has recently benefited from two £1 million donations from Euromillions winners Chris and Colin Weir, and from the estate of late Scots Makar Edwin Morgan, which they have pledged to use towards their independence war chest.

The pro-Union Better Together campaign has also set out to raise "substantial sums" and has pledged to make its spending "completely transparent".

When asked if Scotland could thrive as a small independent country, Mr Salmond said: "I don't think bigness is any security. Spain is a big country but that doesn't make it secure economically. Scotland, when it becomes independent, will be the sixth most prosperous country, in terms of wealth-per-head in the world, in the OECD."

Mr Salmond predicted that Scotland's fortunes may be boosted by an oil price surge in the coming years.

"People say oil goes up and down. It usually goes up in my experience. We're looking at a scenario where oil prices are likely to be north of 100 (a barrel) and maybe even greater than that.

"A lot of people are speculating a further oil price surge, given that we're talking 112 (a barrel) when the world economy is recovering from a recession, when the Chinese economy is sluggishly growing at 8% or something. What would the price of oil be as the economy recovers?"

Mr Salmond also revealed that he has given Chicago's Medinah Country Club, which hosted the recent Ryder Cup, a copy of a 15th century Scottish Act restricting golf.

"Golf was becoming so popular in 15th century Scotland that Parliament passed an act to restrict golf play because people weren't focusing on their archery to defend ourselves against our friends and neighbours to the south," he said.

"As part of an independent Scotland, there isn't the slightest possibility that the Scottish Parliament will restrict golf."

A Better Together spokesman said: "The First Minister can't be a player and the referee in this particular game. On the one hand, he is raising millions for his own war chest and on the other he is trying to dictate what both sides should be allowed to do with any monies that they raise.

"This is far too important an issue and far too important a debate to be decided or influenced by someone who has a vested interest in the outcome. We would much prefer that the whole issue of campaign funding is left to the independent Electoral Commission to decide.

"We will be raising money over the next two years from people who support our cause. The guidelines on what we are allowed to do with those funds should be set by the experts, not the person with the most to gain."